the military presence seemed to be organised as a security detail, not an extermination army, they seemed to have one proper military ship and a hand full of heavily armed transports, they were not equipped to take on the whole planet, and thus they were not able to do that when things turned bad for them. I can see the humans coming back with some proper orbital bombardment weaponry. the Na'vi had better learn to build orbital defences in the next 12 years or **** is gonna get bad.
the only thing I didn't like was when all the animals in the forrest started fighting with them, other than that they did a surprisingly good job of telling this old tired plot.
the aliens were surprisingly biologicaly consistent, the Na'vi themselves being the biggest exception to this.