I have just read the best argument for why Robert E. Lee respected McClellan so much, and it comes from a wise neighbor to the north:
Sorry to be late. to this, but here's my bits of copper: Both men would
have studied classical warfare in their training, especially the wars
of Rome. With McClellan delaying, avoiding battle and waiting for the
naval blockade to do its work, Lee may have seen him playing a Fabius
Maximus to his Hannibal.
That makes a lot of sense. In the Second Punic War, Hannibal was always looking for the final knockout blow against the Romans, forcing them to capitulate before their seemingly endless reserves could tip the scales back against Carthage.
I have read that Lee expressly wanted to duplicate Cannae, Hannibal's great (if not quite war-winning) battle of annihilation. Whatever else his failings, McClellan never gave him that opportunity.