It isn't clear that Kerry's outburst is a positive sign. In fact some think it's going to be quite a turnoff for voters especially since
1) Bush personally has spoke positively on Kerry's service in Vietnam
2) Kerry is now reigniting a story that had just started dying.
3) Bush had a fairly positive outlook in his final speech and going negative might have a backlash effect.
As far as Kerry being a strong closer, as Kausfiles says, he hasn't shown he's a strong closer anywhere that isn't strongly Democratic. Actually what does it say that he's had to be a strong closer given how Democratic Mass. is?