Sure, you can point to the likes of Rooney, Tevez, Toure (who let's not forget all of which are getting roughly 250k per week to play for their clubs... something that would make a huge difference in terms of commitment I'm sure) but what about the others? Even talking about those, I'm sure if you put them in a team who has been in our position the last five years (never strengthening, trophyless, nothing looking like changing) you'd see a change in attitude.
We put our players in systems that blatantly don't work and in a lot of cases play them out of position (Bendtner, Nasri, Diaby/Denilson, Eboue, Arshavin etc) and don't fix problems which every man and his dog can point out, and then blame those players for 'wanting away'. They want away because of the above reasons.
Fix the root problem (lack of quality in the squad / lack of capability to win things) and you could guarantee a lot more from those players. Why else does everyone on here believe Fergie could get a lot more out of our squad?