The only problem I see with the Combat Advantage being granted by the unarmed opponent is that it doesn't seem to fit every situation, i.e.
Freddy Fighter is minding his own business when a drunk farmer attacks him in a bar. The drunk farmer hefts his hoe, while Freddy, who's been a trained Fighter for years now, wants nothing to do with the farmer...but since the farmer's blocking the only way out, he has no choice to lay him out with his fists.
Something like that. Why would a drunken farmer, although armed, cause Freddy to grant combat advantage?
Also, are enemies without arms considered unarmed? Not being snarkey, there, but it all depends on where we draw the line. Mainly, it'll be DM discretion, but we ARE trying to simplify things.