One of George Bush's biggest advantages over Al Gore in the 2000 debates was the fact that Bush is comfortable in his own skin, while Gore couldn't figure out what color he wanted his to be (see Kerry reprise, below). Gore spent a lot of time focus-grouping between debates to see how he could improve his acting, which left Bush free to just be himself.
I think one overlooked aspect of Kerry's decision to switch to retro-Iraq-cum-Vietnam attack-mode is that that is who he is, and always has been. This kind of grandstanding and showboating on a war is how Kerry made his name initially, and it's what he's comfortable doing. These are lines he's been delivering all his life, and it's probably even refreshing to return to his roots this way.
I'm afraid this may neutralize some of the President's advantage in this department. People may perceive this as a hitherto-unseen strength and confidence. With Kerry not worrying about how close to get to Bush, or whether to dress in earth tones or a blue suit (and the Oranqe-and-Blue Broncos colors will be terrif for next week's Colorado visit), he can just be himself, and let loose with both barrels.
Of course, there is the problem that himself isn't very appealing.