Norton wasn't difficult to work with because he's egotistical or just a bit of a clown, he's widely known as a perfectionist and there was a lot of his personal input which went into the whole making of TIH - hence his unwillingness to yield to Marvel on certain issues. As you all know, a number of scenes which were in his version of the script were deleted from the final cut and having watched these on the DVD, I felt they added extra depth to the Banner character in the film and showed Norton having a clear understanding of Banner's motivations and also his angst.
Marvel now want an actor who will simply do what he's told, yet by definition that means we will likely get someone who simply does not have the intense interest and care in the character that Norton developed.
From an acting standpoint, I personally thought Norton played the Banner role very well - he came across as quiet, intelligent, disciplined, determined and also phsyically very close to how Banner is portrayed in most media. Bana didn't do a bad job in the Ang Lee Hulk, but in comparison Norton absolutely nailed it.
Norton's response to Marvel's decision was, as I would expect of him, gracious and diplomatic:
