http://www.youtube.com/watch?v=XHEYcHmNDX0 I say Clark fills a lot of gaps for Obama.
1) Second to only HRC herself, he's the best person to bring HRC former supporters to the Obama camp.
2) Probably the best Foreign Policy icon currently available on both sides of the aisle.
3) He's a white guy. Face it, the odds the #2 spot will be filled by either a minority or a woman is very, very low. Perhaps for good reason (whether we like it or not).
4) He's not a Senator. Again, two Senators on the ticket would be pushing our luck too much.
There are probably several more I've overlooked at the moment. Feel free to add your own.