Yeah, it's definitely the character development. Mangels said that the worldbuilding and politics weren't his strengths, and that's all that Martin does without him - he makes worlds that could be interesting, and he keeps track of all the logistics. But Mangels would do all the character development, all the scenes that made each book feel human.
Just for accuracy's sake: Here's what Mangels actually said the last time he commented on their collaboration here.
Re: him pitching novels:
Don't know if he has actively pitched prose since the Mangels/Martin days, but I seem to remember an interview a year or two ago where he said he would be interested to write more if the right projects came about.
He has actively pitched a Titan comic (mini-?)series together with Tim Russ a few years ago , but IDW wasn't interested.