For the most part, I tend to form strong first impressions. It's likely due to the fact that on the occasions when I have looked past my initial reactions to people and given them the benefit of the doubt, I wind up only confirming my first impulse to avoid them.
Not that I have a huge list of things I won't accept in people. It's more a general sense of the person, a gut feeling type of thing when I first meet them. It's difficult to explain exactly what it is, but I always follow my instincts. I've gotten pretty good at being able to see past a lot of initial fronts people may put up and get a sense of what kind of person they are. Then again, I've had a lot of experience doing that.
I'm proabably a little more sociable as far as giving people a chance online than I am in person since I can't really guage someones personality from simple posts as quickly as I do when I can see the persons mannerisms and such.
As far as playing up or downplaying certain of my own qualities, I used to do that. I don't anymore. These days I couldn't care less when I meet someone whether they like me or don't. I am who I am, and people can take it or leave it. Not that I am rude or arrogant or anything, but I refuse to dress, act, or present my personality differently depending on who I'm with at the time. If anyone ever met me in person, they could rest assured that whatever they get the first meeting will be exactly what they'd get after two years of hanging around with me. I'm too damn old to be trying to impress people when they will find out the things I'd be trying to hide anyway in time. Not as old as Gwally (

) but still, I've found just being myself the easiest way to avoid a lot of hassle and heartaches. Besides, I like me dammit, and I'd miss me if I changed
@fas, I can't speak for obsidian, but personally, I don't think of it that way. I can make allowances for bad days, moodiness, etc., but certain things in the way people carry themselves, mannerisms, expressions, etc tell me a lot about people. It's not so much the mood they may be in as certain underlying things about them that they tend to project that are independant of mood or anything else. For example, if all they want to do is run down everyone else they know, I avoid them like the plague. If a guy is talking trash about all his friends to me, then he's talking trash about me to them. On the inter gender side, if a woman I meet happens to magically have every single interest that I do, like everything that I like, and dislike everything that I dislike, or has no opinions on anything that contradict any of mine, then she's faking herself. The other things I can't explain, but some people I meet just send out vibes that tell me to be wary for one reason or another.