There's no way i can give a good reason in 750 characters...
1. great visuals, james cameron is well known for his imagination and his gift at presenting it well.
2. but the plotline was rushed because of the overwhelming yet insignificant scenes of pandora. despite being almost 3 hours long
3. acting was poor (as it usually is now days), and the only good members of the cast were killed off prematurely for no reason and quickly forgotten. what a waste!
4. not a good use of 3d, and not worth the headache.