I think both candidates are hewing to a strategy of under-promise and over-deliver. Makes sense to me.
Here's an interesting article from The Daily Beast that focuses on the "big lines" that clinched previous presidential and vice-presidential debates, lines like "There you go again." (Ronald Reagan to Jimmy Carter) and "Senator, you're no Jack Kennedy." (Lloyd Bentsen to Dan Quayle). The article suggests that there's more to these debates than just the talking points; there's the strategy of goading your opponent into handing you a "gotcha" line.
I'm going to go out on a limb here and predict that Obama will scorch Romney, three out of three. Obama's a deep thinker and he's got a good head on his shoulders. I have no doubt that Romney will hew to a few well-worn factoids that will resonate with his base, but I doubt he'll have the depth of reason behind him that Obama has. And since the debates will likely have the most impact on the undecideds who, I believe, are more likely to be thoughtful voters looking for additional insights, I think Obama will put more points on the board.