Avatar has hardly any plot, and what there is has as many holes as a good Gruyere. Characterisation is thin, in fact the word cardboard would come to mind if they weren't all electronic. The 3D adds little enhancement, certainly not enough to outweigh the disadvantage of having very heavy solid plastic specs slipping down my nose throughout. The half arsed green message just about keeps itself above being ridiculous.
Having said all that, I loved it. It's just a great simple story, with magnificent effects, and in places some pretty decent music. It's all about the effects, and if that's what you go for, fine. If you want something a bit highbrow, don't even think about Avatar - that's not what it's for. If you want a rip roaring ride, go for it. I did - it was great.