A few things, according to Nate Silver: it made more voters feel that Obama is a safe president:
In today’s ABC News poll, which actually had McCain gaining back some ground overall, 55 percent of voters said they considered Obama a "safe" choice, as opposed to 50 percent for McCain. In mid-June, the last time that ABC had asked this question, McCain led in this category 57-50.
The erratic, desperate nature of McCain undermined him, as it should have. And it boosted Obama’s supports among Democrats – winning over the Clintonistas – and pushing him to 69 percent support from 60. That’s a big deal, and I bet it helps explain the big surge in Florida.
The Dish 