An archtype is a term invented by the famous Carl Jung who is also well known for other terms such as extraverts and introverts. Basically an archtype is a template for certain types of personalities that are easily recognized. I think we have quite a few when it comes to 40k. Here are some off the top of my head:
Fanboy - This type loves all things GW. They rarely ever have a bad thing to say about GW. It seems like most Space Marine players are lumped into this category. We all know that Space Marines have the most armies/codices, models, everything plastic, most of the Black Library 40k novels are about Space Marines... blah, blah, blah. It's unfair in my opinion but apparently a lot of people that play other armies feel this way. What I find funny is that a lot of the people that don't play Space Marines eschew a prevalent stance that Chaos Space Marines don't fall into this camp. CSM are basically warp enhanced veteran Space Marines. To me this is a contradiction but that is just the way it is.
Always unhappy about something - This is the player on the opposite end of the spectrum from the fan boy and they always have something to complain about that is bothering them. This is a very broad category. It seems like most anything and everything gets under their skin and they always have an axe to grind. It might be the latest price hike, White Dwarf sucks, why oh why were Squats dropped, etc. ad nauseum. This camp is very vocal online and make their position known to everyone. They are not happy so should anybody else be?
I find myself somewhere in the middle. I've been playing MEQ for over 10 years now and finally put together an army that is not another CSM/SM chapter. I'm really excited about playing daemons as you can all probably tell. So while I am sympathetic to all things power armor I am also able to see the other side of the proverbial story. I do get fed up with the nonstop bitchfests and butthurt we all read on the Internet... To me if the game makes you that unhappy then maybe it's time to find something else to play. Certainly there are a lot of alternatives available. I've tried my hand at a few other games and I have found them all to have just as many perceived problems as 40k, if not more. Warmachine plays too much like a card game. I did really like the version of Starship Troopers with the rules written by Andy Chambers... It is pretty darn awesome and circumvented some of the problems with 40k; for example anytime you want you can measure any distance on the table. The human jump infantry play(ed) a lot like the Tau - sort of a cross between Fire Warriors and battlesuits. I played a couple of demo games and even bought some models but at the end of the day whenever it comes to starting up a new gaming system I drop it because I am a hardline advocate of 40k... I just can't bring myself to put time and energy into another system that could be funneled into 40k.
There are other archtypes as well but I think these fall under subsets of the first two I listed. You have people that advocate strong comp rules and then there are those that want no comp at all. I think people go through a phase of calling armies cheesy that they can't beat during their early years as a gamer. Some people seem to get stuck in this mode though unfortunately. To me an army that is hard for mecto beat is a challenge and typically I welcome it. If all our games are nothing more than an exercise in clubbing baby seals then we will never grow as master strategists and veteran gamers. I understand that some people say for them the game is purely a social venue but when you get right down to it I think everyone really enjoys the feeling that comes with winning a game. Personally I feel that people who play purely to socialize should refrain from playing in tournaments as they seem to have a tendency to ruin it for everyone else. It's no fun for either player when one of the two constantly berated the other player's army for whatever reason.
What's really annoying are those people who get stuck in one edition of the game and cannot adapt to the next edition. I remember a forum that was solely dedicated to 2nd edition well into the end of 3rd edition. It was hilarious to read their rants and I'll leave it at that for now.