Whining in General: We get it. You don't want to go to work. You don't want to do your homework. You can't wait for classes to end. Now stop posting about it on Facebook before I hide you from my newsfeed.
Passing Judgement: A lot of my friends tend to judge people merely by their appearance or by what they've heard. Heck, even sexuality and religion. I don't understand that at all. People can't help how they look, and people make mistakes. They have their feelings and they have their beliefs. And like it or not, everyone has a few skeletons in their closet. Yet I constantly see people write others off with no valid justification. Why is that so hard for some people to understand that doing something like that isn't fair?
Also...
Trash talk: I don't understand this at all. Any time you score a point in some game or sport, and feel the need to boast about it, often of the "in your face" way, I just don't understand how anyone can rub it in like that. You've got your personal validation, now go home with it. You're just setting yourself up for hatred now and later.
Not giving it your all when it's just really not that hard, or can improve yourself by it: You have two hands. You have two shift keys. You really can capitalize that i you know. It won't kill you. If you pulled that crap on an essay some college professors would string you up and hang you from the flag pole.
Whining for attention, then keeping it to yourself: Posting on Facebook with "Today was HORRIBLE." Then getting a reply, "Oh? Sorry to hear. What happened?" Then responding with "I don't want to talk about it. Mind your damn business." People do this all the time. They set themselves up for attention from their friends and people who care about them, and then shoo them away as if their concern means nothing.
Theeeese.
Online multiplayer: When you're playing with your friends it's ok, but I don't know how so many people can handle playing with strangers. I can play games like Guitar Hero because your opponent has no influence on your actions, they can't talk to you, and if they're cheating it's no skin off my back - I can leave and find another opponent. The only thing that annoys me is people that quit because they're losing (which is most of the time, since I'm so 1337). Anyway, I digress. Other people on the internet are assholes. Period. No qualifiers necessary - if you are on the internet, and you are not me, you are an asshole. I'm not going to play a game where I'm forced to put up with assholes, and I can't understand how so many people do... oh right, because they're the ones being assholes.
Poor orthography (yes, it's totally popular): Learn the difference between their, there, and they're, and between your and you're (it's and its is slightly more bearable). Use punctuation, and use it where appropriate. Make also ur grammar gud.
As a special case, I can't stand people that say "I could care less". The saying is "I couldn't care less", as in "my care for this subject is as low as it could possibly be, and couldn't be any lower". Saying "I could care less" implies some degree of care, since it's possible to care less than you currently do.
And theese.