Of course the movie only shows a handful of technological advancements over the present day, which makes its supposed setting, 140 years from now, pretty unrealistic. But there's the anthropic principle of popular movies to consider: the masses must be able to identify with (and mostly understand) what's going on, in order for a feature to be popular. There's a reason high concept 'hard' sci-fi novels (feasible imaginings of our future) are such a niche market, making a Hollywood budget film that emulates this literary genre would be financial suicide.
However, unlike most movies shoehorned into the sci-fi genre, Avatar appears to have been *very* carefully crafted to make the necessary 'anthropic compromise' plausible (for those who care to think about such technicalities; myself and you). Of course, if one understands the consequences of accelerating returns, you still just have to ignore the lunacy of the time frame; it's there to help suspend the disbelief of the ignorant majority.
Cameron has deliberately found context that justify the compromised details, working around some explicitly in dialogue and leaving hints about others lingering in the background details:
