And ye shall know him by his army
Have you ever found that there's a trend amongst players of any given army? It's crazy, and it tends to be a tiny percentage who just happen to end up colouring the whole group, but I've frequently noticed trends amongst the playerbases of each faction.
(Note: If your army gets a less than stellar overview - which anyone who's ever read anything I've written before knows is inevitable - I don't mean it's about you yourself. It's a fairly tongue in cheek article, for your delight and delectation and not to be taken seriously. It's a piece of fun I wrote because sometimes I like to write huge battleships of text, but I'm not in the mood for a tactica).
Blood Angels players have steadily gotten closer and closer to falling to the Black Rage every edition; in 5th, there were lots of them who were pretty chill (lots of fluff gamers; in fact, mostly fluff gamers other than the people playing their Space Marines as BA because of the 'better' Codex), but in 7th are very much in the running with CSM players for the 'Saltiest fans' crown (given the gradually deteriorating love from GW culminating in the 7th ed BA book, I don't really blame them...)
Dark Angels have been through phases. In 5th, they were 'players who really like Terminators', because that was all their Codex had going for them (even the hardcoriest of hardcore fluff gamers seemed to just run Codex: Space Marines and ally in Deathwing because that was a thing you could do). 6th saw a few of the really dedicated fluff players move back to playing with them, and the odd Banner of Devastation army zooming in to collapse a table under the weight of their dakka dice before zooming off again, but generally speaking the few who played the faction were nice guys, cheerfully optimistic about the few fancy tricks the army could do although occasionally annoying when they argued that their Codex was the greatest thing evar. With their 7th ed Codex they've returned to what some might say is their rightful place as pretty much Marines for people with a plasma and Terminator fetish. As such, it's hard to make an overall judgement; the Marine fanbase is ubiquitous and all-encompassing.
Grey Knights players used to be horrendous in 5th, when they swaggered around convinced that their army was the greatest thing since sliced bread and that even unpacking it from the case would cause all lesser armies to physically fly from the table in shameful defeat. Come their 7th edition Codex, they became a whole lot more agreeable; generally, you play silver Space Marines because you can't get off without back-to-backing The Exorcist and Constantine as foreplay, rather than because you picked the army the internet thought broke the game.
The Space Wolf player is, thematically enough, rabidly loyal to his army and won't hear a word spoken against the army, the models, Leman Russ or any scene in any Black Library book that contains the Space Wolves. Other than this minor quirk, I've been unable to establish a consistent trend. Some Wolves are bro-tier, some are total assholes.
Regular Marine players are infinite and eternal. They are the homogenous grey blob. You'll find a Space Marines player for every different archetype you can think of, and some you can't.
Imperial Guard players, in my somewhat limited experience of them, don't tend to know the first thing about the game. Maybe that's why Guard don't place in tournaments. It's an odd quirk of the universe that of the players I've met, it's almost always the Guard player who doesn't know the basic rules of the game. Of all my oh-so-sweeping generalisations, this is probably the one warped to the greatest extent by my own experiences.
Literally never met a Chaos Daemons player.
The Chaos Space Marine player is feared across he community, for the power of his salted rage is mighty. There are a couple of particularly noticeable archetypes; the guys who are in it because ten thousand year old traitors performing daemonic rituals are awesome (note: these players are also awesome), the Great Salty Ones who cock up every thread with a 'Chaos Space Marines are bad' derailment, and the Iron Warriors players who totally, definitely play Iron Warriors because they're cool and, er, Perturabo was like an interesting and developed character, NOT because Iron Warriors were the OP secret sauce of the Holy Tome of 3.5ed. Definitely not. They do keep a silver basilisk under their pillow for some reason though.
Dark Eldar are played purely by nice guys who lull you into a false sense of security, working off of the presumption the internet's given everyone that Dark Eldar are somehow bad, and then deliver viciously brutal beatdowns on you just when you'd decided what you were going to do with your tournament winnings. And they still manage to be nice guys about it all.
Eldar have always been played by guys who think they're God's own gift to 40k. Granted, they do know the rules like the backs of their slender, pale hands, but man do they let you know it. Amusingly, lots of them also suck (exacerbated in 7th ed by many bandwagon Eldar using a strong Codex and auto-take units to make up for their failings as players). Back in 5th edition they would Movement Phase the hell out of you and could instantly recall any given result on the vehicle damage chart. All the big fish on 40k I know are Eldar players - it's a big thing for an Imperial player to have a Warhound, but Eldar players with 30000pts of stuff including three Revenants and a Phantom seem to be a dime a dozen. Lots of them also played Fantasy and/or other game systems.
I only know one Necron player, and he's just a really nice guy struggling with a dull-as-mud Codex that facerolls everybody which makes everyone hate playing him.
Orks are the marmite of 40k players. There are some who are spectacularly creative, challenging opponents, and all-round great guys. There are an equal amount of boorish, loud-mouthed gorillas that think making gun and engine noises and yelling incoherently is acceptable in a public environment. The former are awesome. The latter are not. Kinda like Guard in that they tend not to be played by 40k experts, but not to the same degree.
Tau are almost universally played by teenagers who like anime, but about 1% of them use the Tau codex to it's internet horror story extent so are actually thoroughly agreeable gamers.
Tyranids are generally a far more agreeable bunch of people than Blood Angels and Chaos Space Marines players, being resigned to having shit-awful rules until the end of time and making the most of it. Never seen an 'okay' Tyranid army - they're either laughably terrible or absolute works of art, but seldom in the middle.