The workaholic. They show up a week early and build infrastructure dusk til dawn (because working when the sun is up is stupid). They do logistics for two theme camps and drive the big cargo truck for all the largest projects. If you see them sitting down, you stop to ask what's wrong, because obviously they wouldn't voluntarily stop accomplishing things.
The volunteer. They are at the burn the whole weekend and spend most of it looking for ways to help. They aren't necessarily doing the hard jobs, just doing the jobs that need doing, even when that's just a warm body in a seat for 6 hours. They walk up at random times and ask "Can I help with that?".
The camp leader. They herd cats. If you see 40 people camping with each other, sharing a kitchen and fire pit, communally providing entertainment or sustenance or anything else to the burn, this person is somewhere in the background. They know where the trash bags are. They know who is on early arrival and who is staying late for LNT. They know who that third propane tank belongs to, and what color tent the person with the smoke machine is sleeping in.
The artist. They made something(s) awesome, and whether it's a hundred feet tall or a pair of earrings, when you see it you're going to know someone put time and skill into it. They might not take credit for it, but their friends will tell you how much of their soul went into it.
The gifter. They made or brought something, usually a whole bunch of somethings, and want you to have them. They might leave them in a pile or on a table, or they might hand them to you when you cross paths. You're going to remember them when you use their gift, or when you find it in your pocket two days or two weeks and a load of laundry later.
The performer. They can do something, and do it well (or at least enthusiastically). It's something fun to watch, and wherever they do it you can be sure there will be folks in attendance.
The altered state. They took something. They might even know what it was. Whatever it was, their world is a lot more interesting than yours right now. Depending on what it was, they probably want to tell you about all that interestingness. Or not.
The spectator. They are here to experience the event, in whole or in part. They didn't really bring anything to share, physically or otherwise. They want to see and hear, not do.
The bartender. You don't have any alcohol? They can fix that! They have some blue stuff and some brown stuff, and you better have a cup!
The cook. This one is like the bartender, except they looks at their offerings more often and their visitors less often.
The babysitter. If you were in a car, they would be the designated driver. Instead, you're drunk, or passing out, or doing crazy things, and it's their (often self-appointed) job to make sure you don't get into too much trouble.
The sparklepony. They forgot to bring a hat. And food. Someone will take care of them, and hopefully educate them if they are going to return. If you're feeling particularly parental, that someone might be you.
The photographer/videographer. All that shit you missed? They are why you can still see it later.
The storyteller. You're going to end up around a fire at some point. Someone is going to have your rapt attention. It's probably going to be this person. They might not be the most interesting person in the world, but they are close enough for tonight.
The noisy neighbors. At least one of them is not quiet during sex, and you're going to get plenty of evidence in support of that conclusion. They might respond to heckling.
The festival-goer. They attend a lot of outdoor events, mostly music-oriented. They are fun to hang out with, but probably haven't been exposed to the 10 principles, and almost certainly don't embrace some of them. To them, a burn is just another festival, with more free stuff and less organization.
The asshole. The nature of the community leads to some people staying involved who aren't particularly nice to most other people. Their radical self expression ends just shy of your face, and well past your eardrums.
The parents. They brought their kids, who they are probably raising in a way that is somewhat outside the norm. Most of the people they encounter will prove this to be a good plan.
The early departure. Something went horribly wrong, and they are leaving. They might want to tell you why, and you might be able to fix it. If not, they are probably not going to have many good thoughts about the event in the future. Hopefully, that's not the case.
Only a few of these are mutually exclusive. Most people are examples of multiple of these stereotypes to varying degrees. There are certainly many more stereotypes as well. I encourage people to reply with their additions to this list. I'll probably re-publish it in a few weeks with suggested updates and additional entries.