Besides the obvious fact that choosing Ryan makes winning that much harder for Rmoney, I'm thrilled with the pick for another reason. For the first time in many a year, this election will place conservatism front and center. No one will be able to claim that this is a mushy-center ticket. As such, beating Romney/Ryan can and should be spun as a referendum against conservatism as an ideology.
If Democrats are smart, they won't let this election be only about Romney the man (although they shouldn't let up about his taxes!). As awful as he is, this is a chance to land a blow that could put conservatism on the mat. Since the Ryan Budget is unambiguously conservative, Romney has made it impossible to run away from the Tea Party worldview (whether or not he personally accepts it is irrelevant) and he can no longer tack to the center.
Obama has already done a good job articulating the difference in the vision between the two parties. He has used mostly ridicule and humor to do so, to very good effect I think. But now with Ryan, I expect every House Democrat running this year to be shouting from the rooftops about the nature of that difference. And if we keep the Senate and manage to take back the House, a very strong argument can and should be made that what really got defeated was conservatism. I'll be satisfied with keeping the White House and Senate, but if all the stars align, this election could do for progressivism what Reagan's victory did for the conservative movement in 1980. And picking Ryan is what made this possible.