It's so annoying to watch him 'run' at a defender to just make a full stop and stand there when he's approaching him. I think it has a lot to do with the fact that he only uses 1 foot which limits the options to (A) get passed the defender on the outside or (B) pass back. If he would actually cut back from time to time to cross it with his left, it would be so much harder for a defender to mark him. Right now, Valencia just stands there and the defender knows he only has to defend the outside so he Valencia keeps standing there hoping the defender will somehow 'take the bait', which they just don't do anymore, so it results in standing there for 10 seconds to cut back and then pass back without any threat for the opposition.
Cross it with your left, Tony! Surprise them! Be unpredictable! It's what a winger should be.
Probably the most annoying thing for me is the fact that during his United years, I remember only 2 occassions where he used his left foot. One was a (decent!) cross and the other one was this one (at 0:40):
http://www.youtube.com/watch?v=A8spkx1Xim8
So it's not that he can't do it at all. It's just that he chooses not to use his left foot which limits his game tremendously.