How-to-draw tutorials/"I can do better than that"-type comments from people who, quite frankly, suck.
It is okay to suck. Feel free to suck all you like in your quest to acheive non-suckiness. You don't have to beat yourself up over it either (I choose to, but that's my nature). You can suck and admit to it, then go "But I'm learning and getting better" (assuming you are). And that is well and good.
It is not okay to suck ass and think you are god's gift to art. When you finally learn otherwise, it will break your brain.
Ditto for people who say things like "You suck, I can do so much better than that" and respond with "Well, my best stuff isn't online/scanned in" when you point out the many, many flaws in their work. If you can't back up your claim, don't say anything.
[thought I should clarify]
Objectively pointing out things that need improving does not apply here. If you go through and say "Well, this needs fixing because people don't bend like that; I can't tell where the light source is coming from," etc. then hurrah and I love you. If all you have to say is "h4h4 suXX0rs!" then I will hunt you down and eat your soul.