Hm...I think they're all bad, but some of them just sounds somewhat ridiculous for me.
1. Well...tough luck. They don't know, what they're missing.
2. That one sounds a bit disappointing, but I think, that all artists have to get through this once in a while. They have to learn that sometimes people don't appreciate the pieces of art you worked the most on.
3. Those, who say/write such things without any logical explanation are just plain assholes. If they have some kind of a thorough explanation about why they don't like your character...then it should be considered a critique.
4. The same goes here, but I guess they are also somewhat dumb.
Haven't they figured out, that if they are using online interfaces then someone's always happen to stumble upon them sooner or later?
5. Hm...It depends on the situation. If the commenters are writing bad things about you, then the same goes here as the above. If your friend also says those bad things... I dunno if someone can call that person a friend anymore.
6. Well... that takes the "Being an asshole" meter to a whole new level. I've been a victim to this one though, and I can tell you, this "trick" is mainly used by typical forum trolls.
7. That's also a "tough luck" situation. The only thing you could do is keep practicing, and maybe one they they will accept you for an art trade.
8. Now THAT'S what I consider ridiculous. You see... that's just like adding a friend on facebook or something like that. I don't think anyone should take this one seriously.
9. That one also depends on the situation. If the aforementioned person asked for your approval, and you gave him/her, then it's okay, I think. If not... that's something you should take care of.
10. That's the case of typical art theft. However... actually I don't really consider it a reeeeally bad thing. I mean... if someone steals your art... that's a really big compliment from him/her, because those kind of people always pick, what they consider very good.
Huh... I started making a habit of writing long comments on your polls and journals.