Great read on The Messiah over at The Hardball Times.
I recommend reading the whole article (and some linked posts), but here is the last bit -
Phil Hughes has a huge amount of upside, but there still are concerns. If he continues to not use his slider and change-up, he effectively becomes a two pitch pitcher. Now despite what you might have heard, starters can make it in this league with only two pitches as long as they are both quality pitches and if he can control them well. Hughes' control doesn't seem to be an issue despite his spate of walks this year. Everything that I read makes me think his control will be between good and great. The quality, though, might be an issue.
The fastball that Hughes has shown so far is average at best. He might be able to spice up the movement with a lowered arm angle or make some mechanical adjustments to get back some lost velocity, but if it continues as it is right now, major league batters will hit it hard. His curve has extraordinary movement but maybe too much movement for his own good. If hitters don't have to worry about the slider and the change, identifying the curve will be that much easier because I don't see a good solution to how he can disguise the pitch any better than he already is.
I feel like it is paramount that he throws his slider or change-up more often (preferably both) to keep hitters honest. Even if neither of these pitches are plus pitches right now, the slider at least has that potential. Most of Phil Hughes' future is ahead of him and he should turn into a great pitcher, but he isn't there yet. Time will tell if he makes it or not.