The main reason Joe Biden makes most top ten lists these days is his experience with foreign affairs. Biden is probably the best pick in terms of religious politics. Obama desperately needs to retain his lead among Catholics and improve upon John Kerry's showing. But choosing a pro-choice Catholic could backfire because the Bishops and others will attack him or her for being a bad Catholic. Choosing a full-blown pro-life Catholic would alienate pro-choice, independent women and Hillary voters. Biden is pro-choice but got a low rating from abortion rights groups (60% in 2007, 39% in 2003). In other words, he's Catholic enough to appeal to Catholics, pro-life enough to avoid some Church attacks, and pro-choice enough to satisfy Hillary voters. (For more on the "faith factors" of the other VP nominees, click here).
In fact, now that Biden is the frontrunner, I'd like to hedge my bet with this more radical thought: now that Hillary's been pretty much ruled out, Obama can choose her. Two months ago, despite compelling arguments for Hillary, Obama could not have chosen her even if he'd wanted to. She and her people were pushing so hard that Obama he would have looked weak if he'd chosen her.
Since then, Obama campaign has let drop little hints that she's not being as seriously considered as the others. Most of the lists don't even mention her any more. She seems to have moved on as have most of her people. So, now if Obama were to choose her, he would look strong not weak. Because he doesn't have to choose her, he can.
In fact, come to think of it, the lack of Hillary being included in most of the lists is suspicious. If Obama were really serious about not picking Hillary, wouldn't he be sure to leak that she's being very seriously considered, in order to assuage her supporters? Her absence from the lists is the best sign that she's a strong contender!