The good stuff
Overall, the movie's message is that we could all stand to learn something from a (fictional) peaceful, nature-loving alien population. Strong environmental and pro-peace themes. Some viewers may see the message of occupying a foreign land to usurp their cultural riches as a political dig at America's involvement in the Middle East.
Several characters make difficult but moral choices. Jake chooses to defend the Na'vi even though it's against orders to do so and means he must fight (and kill) fellow human soldiers. Neytiri, Grace, and Trudy all make personal sacrifices to help the clan; they're strong, courageous, and assertive female characters. (In both the human and Na'vi populations, female characters are just as brave and important as the males -- even the Na'vi mating ritual requires that a female accept/choose the male who chooses her.) On the flip side, the Colonel and corporate boss Parker are portrayed as bloodthirsty and greedy.
What to watch out for
Characters (supporting and extras) die due to explosions, bullet wounds, arrows (some treated with toxins), precipitous falls, and asphyxiation. There are several intense scenes involving frightening Pandoran creatures and plants, as well as tension between Jake's rogue group of pro-Na'vi humans and the rest of the humans sent to Pandora.
Characters (supporting and extras) die due to e…
Many longing looks between Jake's avatar and Neytiri, which eventually leads to kissing and "mating" (only kissing and touching are seen on screen). The Na'vi's humanoid bodies are barely dressed.
The word "s--t" is said several times (as well as its brothers, "bull s--t" and "holy s--t"). Other colorful language includes "godd--n," "piss," limp-d--ked," "h--l," "oh my God," "ass," and mild insults like "stupid," "ignorant," etc.
No product placement in the movie, but there are dozens of tie-in merchandising deals tied to the title -- including toys and books aimed at young kids.
Drinking, drugs, & smoking:
Sigourney Weaver's character, Grace, smokes cigarettes and somewhat glamorizes the activity.