Visually engaging. Special effects were used properly as an aid in storytelling, not merely to impress moviegoers or to show "hey, we can do these CGI thing in a cool way!". Pro-green (yay!).
The Na'vi world is so fully realized, so defined, so unique, and so alive. When the Hometree was being cut down and the Na'vi were being forced out of it, it was just cruel.
Of course, the deus ex machina element is hard to miss. That Neytiri happens to be the tribal princess who can get away with bringing in an outsider without so much as a slap in the wrist for being a bad, bad girl is so telling and indicative of how those who are rich, powerful and/or popular can get away with things.
Sure it took itself seriously. Sure, there was yet another love triangle a la Jack/Rose/Cal with Jake, Neytiri and Tsu'Tey. Of course, the script left much to be desired. What can I say?
It's a movie that succeeds in dragging you to a different world. I was entertained enough by the pretty colors. :D