...is that he doesn't appear in the kinds of films I'd like him to appear in more often (Children Of Men, for instance). Instead, he stars in vehicle sludge like Derailed, with Jennifer "OMGI'MOKREALLYSERIOUSLYIDON'TMINDTHATBRADDUMPEDME" Aniston.
It's the same thing with Brad (Pitt). His best roles - Fight Club, 12 Monkeys, True Romance - find him subverting the pretty boy image (although he is a very pretty boy) and moving beyond standard matinee idol fare. He's much better playing an anarchistic genius, mindblown stoner or raving bug-eyed nutter than ultra-smooth career criminal (the hollow Ocean's 11, 12, 13 and, probably, 657).
So the problem with Clive Owen is that he doesn't do what needs to be done to ensure he's up there with the greats. Damn you, Clive Owen. Damn you to hell.