Peter Suderman...

...says it better than I could:

There were moments in last night’s debate when you could almost imagine Mitt Romney being president, or at least playing one on TV. He was pithy. He was prepared. He was poised. He was very nearly presidential. But what he wasn’t, however, was specific—at least not in the ways that matter most. Romney’s performance was just that: a performance, designed to suggest what he might be like as president, but not what he would do.

Do yourself a favor: go read the whole thing...