Biden is a pretty good, safe choice. He brings experience, age, and leadership to the ticket. Picking another young, relatively untested name would only play into one of McCain's few viable lines of attack. It's now harder for McCain to attack an Obama/Biden ticket as some sort of inexperienced rock-star celebrity pair.
Young, idealistic voters ready for a change are already sold on Obama. McCain has made no effort, and has no prayer, of catching them. Obama needed to pick someone older, experienced, and tested to appeal to more middle-of-the-road voters. Biden's worst negative is his beltway insider and Democratic old-guard status cuts away at Obama's "fresh new change" look, but the change voters are already sold --they aren't voting for McCain anyway.
