Barton consistently chokes at important times. Not just getting himself sent off, but simply playing very very badly.
For all he mouths off about England, he's a liability at key moments and a top club team or an international team can't afford to carry someone like that (provided there are more dependable options).
However, if you consider choking to be an attitude problem, on Barton's basic ability, given United's midfield issues, he would probably fit in somewhere between Carrick and Anderson in caliber, in my view.
For what it's worth, I think Barton was very hard done by for the whole Man City thing. Dumb to lash out at Tevez in the first place, but after that, I believed his comments about strategically trying to take a City player off with him.
As ugly as it sounds, when your team's survival in the premier league could depend on getting a result in that one last game, and you know you're going off, very few people would have the will to do something like that for the benefit of their team.
Was it immoral and unsporting? Ordinarily yes, but I'd argue Tevez and Barton both should have gone for the initial incident (both or neither), so if Barton takes a City player off with him, overall it's a fair outcome. Any player who takes an unprovoked dive trying to achieve the same ends is behaving worse, in absolute terms.