-Blatantly bad characterization (aka, not knowing your character from a random dickwad on the street)

This generally happens when someone only sees their character and doesn’t pay attention to him/her to the point where you can actually get inside of the character’s head, think how they would think,(and when I say this, I’m not suggesting that you attempt to “become” your character. Do not blow up my house.) understand not only their mannerisms, but the motivations behind their mannerisms.

I look at it like this: If you were in love with someone, you would want to know everything about them, yes? You want to know their favorite food and why it’s their favorite food. You want to know why they act a certain way or take a certain position on things. You want to know why they have that scar on their knee. What their favorite music is and why, where they come from, environmentally and otherwise. You get the point.

Treat your character like a lover. And no, this doesn’t mean go smut them up. But hey, it happens. Actually, stalk your character. Stalking uncovers secrets. You want these secrets so you can exploit them to your benefit.

Go, right now. Rant about your character, even if it’s completely without structure. Get to know them, understand them, and then compare what you think you know to the hard facts and fix your damn characterization.

Also, as a brief note, a very wise person with whom I no longer speak gave me what is possibly the best advice I've ever received: When it comes to dialogue, if it sounds retarded coming out of your mouth, it's just as awkward on a screen.

The true test: If a person possesses excellent characterization, they can throw their char into the most insane AU that they can muster and still have him/her behave perfectly IC.

Am I perfect? Hell no. Far from it. I see players who pwn my ten paragraphs with one sentence. But I understand what it takes to learn.