He brings big (conservative) ideas to the ticket, has been a leader in trying to fix the long-term systemic problems that threaten America's future, and is a real intellectual (more than a match for the much-vaunted Obama!). He is pro-life, and he has a talent for making complex ideas understandable to regular people.
What's more, he excites the conservative base, has a young, photogenic family, and hails from what looks like it could be a swing state, Wisconsin.
And last, but hardly least, he has the virtue of "pushing Obama's buttons," much like Romney does. His presence on the ticket may push the President into more peevish, unforced "you didn't build that" errors.
Will the Democrats try to play the Mediscare card? Of course they will. But they would have anyway. After they falsely accuse Mitt Romney of having killed Joe Soptic's wife (and previously run ads of Republicans supposedly pushing old people in wheel chairs over the cliff), there's not much they won't do . . . but at least with Paul Ryan, Romney has a running mate who will push back with knowledge and with vigor.
Finally, the only big downside of Romney's choice is that Paul Ryan's leadership will be truly missed in the House of Representatives if Romney-Ryan ticket wins. Could anyone in the Senate possibly have truthfully made the same claim about Joe Biden -- that his leadership was so important that he was missed in the Senate?!
Way to go, Romney!