I can only speak from my own experience, but I was completely drawn into Avatar, and it wasn't just the visuals. The story has been done many times before, and like you guys said, there is nothing wrong with that. But I completely disagree that it was handled poorly. In particular, I believed in all of these characters and just loved spending time with them - particularly Zoe Saldana's Neytiri and Stephen Lang's Colonel (who is one of the most delectably cartoony villians I've seen in a while). I actually think Neytiri is the key to the film, if you are invested in her, you are invested in the Na'vi, and then invested in Sully's evolved mission. At least, that's what happened to me. You guys may have thought Saldana was good but not good enough to save the film, but I thought she was the film, and she worked perfectly for me. I, like some of the other people you've heard from, don't think any excuses need to made for Avatar. It all worked for me, and I'm fine with that.
(I will give Matty this: I was waiting for Sam Worthington to hop on that dinosaur the whole time, and it was awesome)