What's above his shoulders.
More so than any other Top 10 player, he's suspectible to moments where you can't help but yell at the TV "What is wrong with you!" Whether it's the mess at the end of the Almagro match at AO or losing to Stephane Robert first round of a major! His stretch at the French Open and Wimbledon in 2010 shows there's great upside but he's just not consistent with his shots in big matches.
Did you watch the Robert match?
Probably not. So please refrain from commenting on it.
As for the topic at hand:
Better second serve.
Better movement from the far-out forehand side.
A bit more margin on his rally shot.
^All those are not major issues but they're shortcomings that do stop him from making the next step.
Mentally he's not the weakest player in the Top10 even. Sure it'd be awesome if he was as strong mentally as Nadal but looking at the big picture the dude is not the guy to point out to in that area when you have guys like Verdasco and Fognini in the Top50.
An often critique you see a lot of people throwing at him is "Why didn't he beat *inserthigherrankedslamwinningplayerhere" in that match", I guess that's a compliment for Internet standards.
The way I see it, he could only maximize his potential if he manages his hot streaks of form better, that includes winning ugly and other valuable aspects of competitive sport. Few people seem to have forgot of his pre 09 days when every single day on a tennis court for him was either yes or no, with no grey area inbetween and plenty of excuses in case "no" happened. He went from there to being on paper one of the best players in the world and a decent competitor pretty much against anyone right now.
I think he's done well, and he's still improving as a player and a competitor.