I am not sure I agree that the characters are any less developed that many broadly similar movies.
Arthur is his partner - it is a professional business arrangement, he is being well paid to do this job. Eames is also being paid, and decides in the first dream level that it is not worth the risk - so clearly he is only motivated by the money. Yusuf is being paid twice to keep quiet about the dangers of the compounds he has made. Ariadne is being paid, is also fascinated by her new powers as an architect and has started to develop concerns for Cobb and his sanity. Saito is motivated by the power and money it will make his corporation. Robert Fisher as 'the mark' has the most complete back-story outside of Cobb and Mal.
How is this any different than other caper movies - Oceans Eleven, the Italian Job? Inception is very polished and has a ton of subtext thrown in, but substantially the characters have no more or less backstory than other movies about professional criminals.
In my opinion we are drawn in by a movie with very high standards of production, it looks great - the acting is good, the story is well paced and well explained despite its complexity, and we are drawn into the idea of double guessing whether Cobb is still in a dream or not at the end, and we don't quite know enough even at the end. Best big budget Hollywood movie in some time.