Origin Story Time
You probably don't know this, but I owe my start in the wedding business to DJs. You see, I'm also a musician and audio engineer. I've also spent years selling professional audio and lighting systems to venues, houses of worship, touring bands, and of course DJs. So over the years, I've become friends with many DJs. And when I decided to try my hand at wedding photography, I got my first few gigs thanks to some of these friends. Couple this will my years in the wedding business and I have a very good take on what makes a good or bad wedding DJ.
Personality
Rock The Mic
First, I hate the term Wedding DJ. It gives people the misguided idea that what they're hiring is someone to simply play music. What they should be called is a Wedding Entertainer, as that better describes the role they play in your wedding. So the first thing you should look for in a DJ is someone who has the right personality for your wedding.
You could go either way with this really. You may want an extra-extroverted person who charms and delights your guests with lots of interaction and bit of flair. Or you may want someone who is more in the background with a laid back attitude. There's no right or wrong here, just what's right for you. The truly great DJs can do both equally well and know how where on the spectrum to land for the vibe of your wedding. The most important thing here is to communicate what you're looking for with prospective hires. Also, look for reviews and videos of past weddings to get a feel for what they're like both in front of and behind the booth.
Avoid The Button Pusher
What you want to avoid at all costs is what I call the "Button Pusher." I see this all too often sadly. The DJ should also be the MC (Master of Ceremonies), however many wedding DJs never see that as part of their job, or they don't take it very seriously. Simply because you may want a smaller personality for your wedding shouldn't mean your DJ isn't competent with making announcements, directing speakers, keeping the timeline moving, and getting people on the dance floor. I won't go into too much detail here, but if you search for wedding dj horror stories, you'll read all about these types of DJs - who are basically a glorified play button for your wedding playlist.
Music
They're Playing Our Song!
This is actually the easiest and most straightforward aspect of booking a DJ. Make sure you pick a DJ that lines up with your own personal tastes in music, just like you would if you were booking a band. Some DJs are very good at mixing certain genres, but might not do well in others due to their lack of knowledge or enthusiasm for that genre. Ask them to provide you with previous sets they've done so you can get a feel for their style of mixing. Provide them with a list of songs you want to be played and make sure they have them all in their library. With all the subscription services catered to DJs like Beatsource, DJ Pool, and Tidal, there's no excuse to not be able to access just about any song you want now days.
Sound
Gear isn't everything. You don't need to book a DJ who has the most expensive rig in town. Overkill can definitely apply to DJs. But there's also another side of that coin, where a DJ doesn't have sufficient gear to do a great job. Finding someone who knows where that middle ground is will mean you're booking someone who knows what they're doing. Don't be afraid to ask for a gear list so you can evaluate what they'll be bringing. You should also inquire about what kinds of backup gear they bring with them - because even good gear goes down sometimes. I recommend looking up this gear on www.sweetwater.com just so you can see at a high level what they've got.
PA Systems
Being an audio engineer, nothing bugs me more than a wedding DJ using inferior equipment. But I've also seen a lot of DJs do way more than what was needed. There is a happy medium, but it will depend on a few factors. The most important two factors though are the size and layout of the venue and the number of guests that will be attending.
I don't want to go into the math around acoustics, wattage, dispersion patterns, etc. (actually I really do, I just don't want to put you to sleep!) but there are some things to keep in mind.
The Inverse Square Law: This law of physics means that every time you double the distance from your sound source (say 30ft in front of the speaker to 60ft), you'll lose half its volume. So if you're venue is reeeeeeaaaaaaaallllllllyyyyyy long, you're gonna need a bigger PA if you want the people in the back to hear everything.
The Quiet Outdoors: If you're ceremony or reception is outdoors, you're going to need a bigger PA. This is because, without any walls or ceilings to reflect the sound off of, all you'll hear is the direct sound from the loudspeakers. This also means they become more directional - meaning if people are in the line of fire, they may not be able to hear very well.
People Eat Sound: We're all just a bunch of walking meat-based sound absorption panels. This means the more people there are, the bigger the PA system will need to be. My time as a FOH engineer has taught me well that a room will sound completely different when it's empty vs. when there are 200 people in it. Your DJ will need to understand this as well.
You don't need to be an expert in live sound production, but your DJ should be. So make sure you let them know the layout and circumstances of your wedding. Many DJs will have multiple PA systems (or a scalable PA package) they'll recommend based on the factors above. If you're interviewing a DJ and this all seems like a foreign language to them - run.
Bonus Tip: If you're having an outdoor ceremony, ask what their plan is if there's an issue with the power. Some may bring a gas generator, and while that's okay, they're also very loud and smelly. The good ones will have either battery-powered PA speakers like the Bose S1 or JBL EON One, or a battery generator powerful enough to handle their PA for at least an hour.
DJ Controller/Mixer (Well Well Well, How The Turn Tables...)
Don't get too caught up in the actual DJ gear they use, as it's honestly the least important part of their rig to do a wedding. Chances are they'll be using a DJ Controller that connects to DJ software on their computer. Pioneer is the industry standard here, but what's really important is that whatever controller they're using has at least 2 pairs of high-quality balanced line outputs, one for their PA system and one for a recorder for the videographers. This is vital, as the DJ board is going to be your video's main audio source for the ceremony and reception. The other important qualifier will be whether or not the controller/mixer has a professional XLR microphone input (note: many DJs will have a separate audio mixer for this, that is perfectly acceptable).
If you look up their particular controller/mixer online, anything that costs under $500 is a red flag and likely won't have the bare minimum features for doing a wedding.
Microphones
Microphones are a very important part of a DJs kit. Cheap mics sound cheap, but they're also prone to feedback and dropouts. Nothing is worse than your Maid of Honor doing a speech but you only hear 1-in-4 words. Ask them what microphones they're using, then go look them up at www.sweetwater.com. If the wired mic comes in below $100 or the wireless mic system (mic and receiver) is below $400, don't take that gamble. Also, the only acceptable brands for wireless microphones are Shure, Sennheiser, Audio-Technica, and Audix.
Lighting & FX
This is the area that really separates the men from the boys. You might think that a great PA system and the right songs are what's going to get your guests out of their seats and onto the dance floor, but you'd be wrong. Great lighting is what gets people to shake it like a polaroid picture. We're all just moths to a flame really, and we light shiny things.
There should be three sources of lighting at your wedding: the venue, the photographer/videographer, and the DJ. Each one serves a different function, but the most important lighting for the reception will be the DJ's.
Venue Lighting: This is there to provide the ambience and to make sure there's enough light that no one gets hurt. This is a very important aspect when choosing a venue, so make sure you look at what the venue looks like at night before you book it.
Photo/Video Lighting: Our lighting is meant to light specific subjects at specific moments, like you and your significant other during your first dance, or people giving toasts. This will vary depending on who you book, but in my case, it will entail high-quality video lights that I'm able to focus on the subject I choose, some on-camera lights for when we get group dancing shots, and flash if needed (although I try to avoid it as much as possible because flash can be distracting).
DJ Lighting: This lighting is meant to establish the vibe of the room, or at least the dance floor. What that vibe is should be is up to you, but it's important that you know what you're getting.
Types of Lights
Most DJs will have options for lighting packages you can add on while you're booking them - always always always get more lighting than you think you'll need. Even if you don't think you want lots of fancy lights, making sure your DJ will have a sufficient amount of spot and wash lights (usually called Par lights) for a blanket of color will go a long way. Uplights are also an amazing way to transform a boring-looking venue into something pretty magical. If you want your dance floor to feel like a club, get lots of moving heads and fx lighting. Other really awesome lights you may want to consider are things like Gobos or projectors, which you can get customized to have your and your partner's names put into and thrown onto a blank wall.
Non-lighting Effects
Going hand-in-hand with lighting are the atmospheric and practical effects. The most common that everyone is familiar with is the fog machine. Be very weary about fog machines though, as the chemicals in the fog fluid can be very dangerous to people with asthma or breathing problems. I myself hate fog with a passion. It gives me both a sore throat and a mild headache. If it's thick enough, fog can also set off fire alarms, quite literally raining on your parade. But don't worry, there is a better alternative now to fog... haze. Haze machines are a bit more expensive, so will probably cost a slight premium to rent over fog machines, but it's 100% worth it. Haze is much less harmful, and is far less likely to set off any alarms. Other options you may want to look into are things like bubble machines (watch out for slippery dance floors though), confetti launchers, and sparkler machines. These types of effects can really take your wedding to a whole other level. Black Lights can also be amazing, they mix especially well with glow-in-the-dark objects if you plan on having those at your wedding.
Do Not Use These Lights!
Just like fog machines, which can be harmful to some people, there are other types of lights you'll want to make sure you avoid having your DJ bring to your wedding.
Strobes: Strobes can induce seizures in certain people. They can also be annoying to anyone with light sensitivity, as their eyes don't have time to adjust. They also torture us photographers, as it's like a flash that isn't synced with the camera - they like to ruin a lot of great shots.
Lasers: This is actually in my contract that lasers will not be used at the wedding. The reason is that lasers can actually damage camera equipment. They can essentially burn the sensor, like having a group of pixels go out on a screen, but in reverse. This means the most expensive part of the camera needs to be replaced (generally a $600+ repair). Also, any photos shot after will all have a nice big black spot on them where it was burned. Just say no to lasers.
Halogen... Anything: I don't see this too often anymore, but there are still some old-school DJs out there running around with halogen lights. Halogen is what lights mostly use to run on before LEDs became brighter and cheaper. The main reason you want to avoid these lights is that they get HOT HOT HOT. These lights are essentially heaters that got a minor in lighting. LEDs are brighter, smaller, and infinitely cooler. So save yourself from sweat, and avoid halogen like the plague.
Run DMX
All of these lights and effects connect to either a lighting mixer or software through an ancient digital networking system called 'DMX', which can be very tricky and has a bit of a learning curve. That's why it's important that you get these lights from your DJ, you need someone there who knows what they're doing in order to set them up and run them correctly. This is NOT something you can hand off to just anyone.
Better DJ = Better Pictures/Video
Finally, one of the reasons you should book a great DJ is so you get better pictures and videos. A great DJ will help us out in several ways. We rely on them for a lot of our audio capture, which we pull from their mixer or PA speakers and later align with the pretty terrible audio captured directly from the camera. We'll also try to get audio captured from other sources so we're not solely dependent on the DJ, but no other audio source we get will sound as good as what's coming directly from their booth. The best DJs will understand this and go out of their way to make sure we're taking care of, even going so far as to periodically check our recorders when we're unable to, making sure the batteries haven't died, the levels are not too loud or quiet, and that the red light is still on. And a DJ with an amazing light setup makes our jobs for so much easier. More light will always mean better pictures (less noise, more detail, sharper, better color, better contrast, and better depth). And if they're great at working with a crowd, we'll also get more interesting pictures and video that you'll love for years to come.
Now you know why wedding DJs are so important. So get out there and start booking!