The problem is definitely fit AND in form. He suffers from far too many injuries, and on top of that he needs a series of games to get into shape. He often comes back slightly on the heavy side, and that may also partially explain some of his injuries. Calling him obese, as some do, is plain stupid, but he does carry a little extra weight, and in no way do I think it benefits him with all the muscle he's carrying on the chest, for instance. He should be leaner.
That being said, he's a quality player when fit and in form, but it's far too erratic for considered world class. It's like claiming Saha could have been this or that. The fact is that Saha was never close to Henry's level because of his injuries, just like Shaw isn't close to world class levels because of his injuries. There are many left backs around that I would entertain ahead of him. Robertson, unfortunately, is far ahead of Shaw. I think you need to wear some red tinted glasses to suggest he is amongst the best LBs around. Would be interesting to hear why people think he's world class and none are better?