His tactic in the debates with McCain seemed to be 'do no harm, pass the "look presidential" test'.
He was very reserved, hardly went for the jugular, but always calm and collected, leading many media pundits to consistently remark positively about how he seems completely calculated and informed on the subjects. He appeared very even handed and respectful in his approach to McCain. At the end puntis would say things like "if anyone was expecting the polling landscape to radically change with a debate, this debate was certainly not the one that will do it for McCain." McCain didn't do awful, relatively speaking, but since the expectations game was a certain way, Obama made the right moves for the right time.
The goal during that year was to seem even-tempered and like he knew his stuff, since the "inexperienced" thing was the main attack if we ignore Palin's 'friends with terrorist' attack stuff. And he did it.
I suspect this year, the viciousness of Obama's attacks are going to be far less passive aggressive. He doesn't like Mitt Romney in the least, it has been widely reported, and there is so many ripe low hanging fruit to pick from that the harvest can pretty much be nothing but bountiful for an even half-on-his-game Obama.
And considering that the second debate is a townhall, and how Romney suffers painfully even when he's alone on stage during such a format, and Obama has always been a natural crowd worker, it's going to be a brutalizing.