1. Forced Romance, whether On-Screen or Implied For Later. IF: plot involves man and woman THEN: man and woman will get together for sexytimes, or at least it is obvious they will do so before the credits finish rolling.
Stop this. Seriously, stop it. Relationships begun in the heat of the moment aren't likely to last. The type of sweaty alpha male character you get in many movies isn't automatically the ideal mate for any woman. Also, the situations encountered by movie characters are often awkward, deeply uncomfortable, or even downright dangerous.
Take The Running Man for example. Amber enters the story by being taken hostage....not by the bad guys, but by Ben Richards! She is his victim, not his girlfriend. She seems to have a happy, successful life, until Ben drags her away from it at gunpoint. He steals her money and her travel pass. When the two of them are apprehended, she is thrown into the Zone along with him for the crime of aiding and abetting, which she did not commit. She nearly dies any number of times, suffering great pain and terror.
In the end, the Zone is escaped, Ben's innocence is revealed, the evil mastermind is executed. Ben and Amber are left to enjoy their just rewards, walking out together into the sunset and......
Wait a minute. This total stranger kidnapped you, held you at gunpoint, got you thrown into the Zone, murdered several people right in front of you -- and your reaction is to smile and happily walk out with him? What about your jangled nerves and PTSD? Your need for medical attention after all the abuse you suffered? Heck, what about your shattered reputation and shiny new criminal record? Don't worry, Ben Richards is The Hero. Sex with him will cure all that.
2. Disregard For Physics, usually exemplified by the axioms Explosions Are Slower Than You and He Who Jumps Last Falls Fastest.
3. Forced/Unrealistic Happy Endings. Avatar is a great example. It's been proven again and again that spears and ferocity fare poorly against military hardware and tactics. If guns and helicopters were inferior, nobody would use them. I don't see it being a whole lot different just because the aliens are blue and ride space tigers.
4. Endings That Fall Apart As Soon As You Think About Them. The two movies I already mentioned are great examples.
Are we supposed to believe Ben's criminal record will magically disappear? He murdered a guy on camera. Even if Amber is too stupid to realize it right away, she'll figure out in a day or two that Ben ruined her life. Not on purpose, perhaps, but ruined is ruined. They are NOT going to have a happy, peaceful life together -- or at all.
Are we supposed to believe humans won't come back to Pandora with even more firepower? I certainly would, had I the resources the company seems to. A mineral worth $3M a pound justifies escalation to any level of violence. The blue people have maybe a year to make peace with their planet-deity. After that, they are going down.