Re: @Sarah and Cowherd
I think the characters are fine in context - we know what they want and we can empathise with them and want them to succeed, which is all you really need. It's very basic and it presses your buttons. For more subtle and enriching character development, you go elsewhere. Cameron knows exactly what he's doing with the various elements of film - he's a master.
For seriously bad/non-existent characterisation, check the Star Wars prequels.