Peter Hall at Cinematical:

Avatar is primed to leave a huge dent in the notions that motion capture films equate to soulless, dead-eyed characters, that sci-fi films must consist of pop music soundtracks and giant, clashing robots to have broad appeal, and that big screen fantasy lands have to be anchored in familiar, Earth-like places to yield heartfelt audience investment.

Roger Ebert on his Sun-Times blog:

“Avatar” creates a new world from scratch, and, as Lucas did in “Star Wars,” fills it with such countless minute details that it doesn’t seem artificial. Well it does, but it’s as real as a fantasy can seem, if you see what I mean. The creatures of this planetary forest are many-toothed and preposterous, but not the grotesque artifices of so many monster movies. Their battles seem sincere and earnest, and not the banging of pots and pans as in “Transformers 2.”