Reddit has been ripe for disruption for years. It's just waiting on an inflection point and someone to take it behind the barn.
Remember, the only reason Reddit "won" was because Digg destroyed itself with a radical upgrade that everyone hated.
Reddit would have to do something similarly self-inflicted, and I can't even guess where people would go. Reddit was already an alternative to Digg -- what's the alternative to Reddit? I mean, it's certainly not Quora.