His volleying is crap. Crap is a strong word so I will say his volleying is average. It cost him the match against Nadal. He will never move like the guys in the top 4. Mentally he can never match or keep up with Nadal, Federer and the Djoker. The courts are not helping his game. He has to hit 2 or 3 winners in order to get a point against speed merhants like Nadal. He is pretty much screwed as things stand. The best he can hope for is reaching the last 8 or last 4 in the truly big events. He can spring a surprise here and there but he can never truly disturb the top guys frequently just like Tsonga.
On the contrary crap is not a strong enough word. Other than his exploitable movement (which he can get around with his huge power game if he stays ahead in the points) his lack of variety, particularly at net really hurts him. Sometimes those few points here and there make all the difference in a match.
His return game can be a little depressing at times, as well.