And that's why many (including me) don't think Avatar will win -- the Academy voters seem very suspicious of movies that "wow" them or that are just a good time. Star Wars, E.T., Raiders, Starman--all got nominations but didn't win. Best Picture tends to favor "heavier" movies for lack of a better word.
Also, I think actors are 20% of the voting bloc and a lot of them prefer to be in movies where they mainly act, not do the whole voice/motion capture thing 3 years before they get to see what they were actually doing/acting against. A future full of Avatar-like movies is a future where one's carefully maintained, surgically perfected facial features aren't front and center, and more seriously, one's acting will most likely not get nominated.
One more thing--there's always talk of a movie's Oscar hype "peaking" too early. If there is a movie that that is happening to this year, it's Avatar. It's probably just a bit past the moment when the most people think it's awesome and the best movie ever made and starting down the downslope of backlash/adjustment of initial reaction. When the front runner peaks early, it's a year that the 2nd or 3rd place movie could have a surge. And with 10 in the mix, it's going to be harder to predict how things will turn out. That actually could work in Avatar's favor, since the "anyone but Avatar" votes will be split 9 ways instead of 4. And there's always the exception that proves the rule, as well--Return of the King is a good example. It had every bit of the momentum and although there was a little bit of "peaking" discussion, it had all the momentum right down the stretch,to the point where it would have been literally shocking if anything else had won.
Finally, Cameron has his Oscar already. Titanic cleaned up in 1997*, and other than L.A. Confidential, was probably the most deserving of the Best Picture nominees that year. Fair or not, there does seem to be a tendency in the Academy to want to distribute the awards to people who haven't won before over those who have.
None of this touches on the previous arguments on the actual quality of the movie, just the politics of Oscar. I will be very, very surprised if Avatar wins.
*Forgot to add my footnote! Well, here it is: Just an observation that '97 was a weak-ass year for Best Picture. Titanic and L.A. Confidential I'm down with, but the other three were Good Will Hunting, The Full Monty and As Good As It Gets. Of those, Good Will Hunting was probably the third place movie where in most years it would be in the "smaller movie that could/it's an honor to be nominated" slot. That The Full Monty got in is a shocker; as for As Good As It Gets--ugh. I really disliked that movie.