First off, no, it wasn't crossover Democrats who brought Cantor down. Teabaggerdom is gonna have to own this one, and judging by the triumphant excitement on RedState, they're more than eager to do so.
David Brat has only been famous for a couple of days, but I'm already getting a whiff of Christine O'Donnell. His website implies he attended Princeton University, whereas he actually went to Princeton Theological Seminary, a completely different institution. His core belief system seems to be a combination of Ayn Rand and Bible-thumping. He's already flubbed an interview because he doesn't know anything. What else is going to come out in the long months between now and election day? Will he manage a signature gaffe as memorable as "I'm not a witch"? If so, Democrats may have a fighting chance at a seat Cantor would easily have held. If Brat does get elected, he'll be a loose cannon and a constant threat of embarrassment to the party.
Brat's big issue against Cantor was illegal immigration, an issue on which Cantor was not exactly a flaming liberal -- except in the eyes of teabaggers, whom I've repeatedly seem calling him pretty much exactly that. Odd as it may seem to liberals, the political territory to Cantor's right is vast and well-populated. In a hard-red district like VA-7, those zombie hordes are numerous enough that possessing any trace of a brain can be dangerous. Congressional Republicans, freshly re-terrorized by the revival of the bagger beast they thought tamed at last, will become more timid than ever on this and other issues. And the right wing's internal debate over illegal aliens, and how (or whether) to appeal to Hispanics more broadly, will heat up (see comment thread here, for example), generating plenty of hateful and alienating rhetoric.
It's events like this that keep me somewhat optimistic about November despite the generally gloomy omens. The Republican party is so divided and so full of wackos that a certain number of own goals is inevitable. We can't predict when they'll happen or what form they'll take, but they'll come.