It first really struck me with Vanguards. I'm ridiculously biased in their favour. While I've certainly had some positive experiences with random Vanguard players in the past, none of it even comes close to justifying the sheer extent of my bias. I think it comes down to a mix of class identity and their role in gameplay. The trooper story is very much about being a "good soldier", even if you make dark side choices, so I expect people who play one to find that role appealing, and to enjoy being a hero of the common people. Vanguards then take this another step further by being able to tank, and a lot of them do. In PvP this means that they really are protectors of the common (squishy) people - holy crap, this guy literally just took a
Hello there, handsome stranger! I feel strangely reassured by your presence. Let's be friends?
This got me thinking about how I see other classes. Interestingly, I expect Commandos to be kind of derpy, even though they are "noble" troopers as well. Considering that this is my main's class, this might simply be a case of "takes one to know one". One of the Commandos in my old guild is also extremely talented at getting himself into trouble. And for quite a while after release, dps Commandos were widely known as "one button wonders" that were supposedly popular with players that had no skill. I guess this might simply be a case of overwhelming amounts of past experience overriding everything else.
When I see a Scoundrel, I expect him or her to be a easygoing and friendly. This is probably a mix of story and role bias again: smuggling is for people who like to crack jokes and have fun... and somehow most Scoundrels I encounter these days are specced into healing, which makes them instantly seem supportive and helpful.
Gunslingers I view as more of a lone ranger type, sitting at the back behind their cover and doing their thing while nobody else pays attention to them except to say: "Dude, you know you're out of healing range, right?" I also expect them to be very serious about doing dps, because otherwise they wouldn't have chosen one of only two advanced classes that can perform no other role.
The latter is a prejudice that I apply to Sentinels as well, though I also view them as more sociable and helpful. They may be pure damage dealers, but they are still goody two-shoes Jedi, you know? It also seems to me that among all the different damage dealers, Sentinels are the ones most likely to play in a manner that still focuses on supporting other players (for example by using their utility powers at just the right time). I admit that this perception might simply be a result of their high visibility though (hard to miss that jumpy guy with the dual lightsabers who's constantly in your face), and that other dps classes might be just as helpful from range and just not get noticed.
Guardians are - to me anyway - the game's "average guys". It seems a bit wrong to say that about a Jedi class, but basically they represent the typical good guy or hero with no other defining features. If nothing else I expect Guardian players to be well-intentioned, however other than that I pretty much anticipate mediocrity. I know that I'm doing them a great injustice that way because I've known some very good Guardian players in my time, but I've also seen just as many Guardian tanks who can never remember to reapply guard and dpsers who only ever do average damage (this includes my own Guardian alt by the way). I just can't get excited about being grouped with a Guardian either way.
There is another class this applies to for me, and that is Sages. Maybe it's because they were all over the place shortly after release when they were the major flavour of the month, but they always struck me as the kind of character played by someone who just wants to have it easy. (Note that my main alt is a Sage.) I'm always hesitant to give my MvP to a Sage because even if they did well I always doubt whether they really worked for it. I'm always a little suspicious of them initially and they'll have to be truly amazing to impress me.
Now Shadows on the other hand, I expect those to be highly skilled players, I'm just never sure about their attitude. They seem to get played by people of extremes, and will either be wonderful team players that provide great utility, or selfish jerks who constantly roll their eyes at all those other people who can't stealth and slow them down. I think this might be a prejudice that is related to stealth dps gameplay in general as I vaguely seem to recall having a similar bias against rogues in WoW back in the day.
I can't really comment on the Imperial advanced classes because I mainly experience them as the enemy, so my biases are pretty much limited to "is annoying", "hurts" and "hurts a lot". However, I think that even if I did play Empire side more, the prejudices I listed here wouldn't automatically translate to their mirror advanced classes due to some classes having a very different flavour to them - I don't think I could ever view a bounty hunter as a heroic protector of the people for example.
Do you expect people to act a certain way based on which class they play?