Graham laid the foundations without which I doubt Wenger would have achieved what he has. For example, Wenger inherited the back 5, so his lack of defensive coaching ability didn't affect us. However, now that Wenger himself has had to construct a defence, we've seen the consequences. Various reports suggest that Wenger barely works on defensive organisation (not just with the back 4, but in terms of midfielders tracking back and coordinating efforts etc) and the repercussions of this is clear whenever you see us concede silly goals through a lack of defensive solidity.
Wenger is very good at the technical side of coaching (in terms of developing a pass and move game, ensuring players are at peak fitness, their diets are optimal etc etc) but he does seem lacking when it comes to the "bread and butter" so to speak. It must also be said that, in recent years, even his much famed ability to develop players has to be questioned. Has he really been able to improve players like Diaby, Denilson, Bendtner, etc as he promised?