Mistake #1: False exposition, or the "as you know, our mother died last week" problem.
Avatar example: (Big bad corporate guy, speaking slowly to hard-boiled scientist woman) "This is why we’re here. Unobtanium. Because this little gray rock sells for twenty million a kilo. No other reason. This is what pays for the party. And it’s what pays for your science."
Viewer reaction: REALLY? You've told me that there are teams and teams of people who have painstakingly grown genetically-specific Avatars, learned a whole new language, and had "years" of troublesome relations with the indigenous people, and the head scientist doesn't know about the MINERAL?
Solution: Do not have your characters tell each other things they already should know. If you have to get this information out in dialogue, use a character who has the same lack-of-knowledge as your reader.
Mistake #2: Painfully obvious stereotypes.
Avatar examples: (Bad corporate guy) "Killing the indigenous looks bad, but there’s one thing shareholders hate more than bad press -- and that’s a bad quarterly statement."
Viewer reaction: SERIOUSLY? You spent all this money on Avatars and all that other crap, but NOW it's cheaper to blow it all up? Then why not blow it up before you got involved with them? And, really, not even the big tobacco guys talk like this. Finally, for those of us who used to handle legal matters, do you know what often makes for a bad quarterly statement? BAD PRESS. People will boycott products -- even ones they love and thought they needed -- if the company is horrible enough.
Solution: Okay, stereotypes sometimes exist for a reason: some corporate guys ARE greedy. But please don't use stereotypes as a shortcut to establish plot or character. The character of Jake plays with some of the "dumb jock" stereotypes quite nicely, after all. (And while we're at it, don't name the unobtainable mineral unobtanium, or the selfish corporate guy Selfridge. Don't even get me started on the "noble savage" thing Cameron did with the Na'vi.)
Mistake #3: False tension.
Avatar example: (Military squad leader who is once again imitating the Full Metal Jacket speech) "You are on Pandora, ladies and gentlemen. Respect that fact every second of every day. Out beyond that fence every living thing that crawls, flies or squats in the mud wants to kill you and eat your eyes for jujubees."
Viewer reaction (as the show progresses): That's really pretty. Wasn't this planet supposed to be horrifyingly deadly? Was the military guy supposed to be lying? No, doesn't seem that way, because there's another reference to the deadly planet. Hey, our hero's leaving his body behind a lot. No danger to it, apparently...
Solution: Don't over-sell. Or explain why your hero is so special that none of the rules apply.