he's actually a very overrated player
he is not particularly powerful and adopts an extremely poor style of play (extremely defensive based on his strengths). sure he's very athletic and fast but what difference does that make if he's running 24/7 and behind in points? occasionally he'll find some form and ballstrike well but that's once in a blue moon, he's not particularly clever or 'there' strategically on court
the US Open loss to Federer this summer says it all; if he was ever capable of going the distance it would have been there, but in the end he clowned that match just like he clowns time and time again. he clowns his career and its not even the entertainment efforts that is the problem, it is that he truly appears unintelligent in terms of tactical play; i.e. appears totally unaware of when to be aggressive, when to play the point more passively, when to go to net. he gets away with a lack of tactics against many players he faces simply because he is athletic and can 'last' in a point but against the best it just doesn't work
what's the strangest fault he has is that while he is fairly quick and appears (???) to be quite fit, his endurance is very mediocre and more often than not he will fade away when matches go the distance
he's got some flashy shots, or moreover, flashy movement at times but just doesn't have the entire package. sure if you could somehow replace his brain with that of Nadal's or Murray's, get him to totally change his tactics/gamestyle then MAYBE you could produce a slam winning formula but we don't live in some magical la-la land where there's scientifically possible. AS IS, how Gael Monfils is now he is not anywhere near capable of winning a slam