The problem is that by introducing a monetary system to vote/share content, you fundamentally change what made reddit successful in the first place: an environment where you can freely speak, regardless of how much other users approve. With the pay-to-vote system, it just quickly become a giant circle-jerk contest of who can say the thing to attract the most upvotes, and who has enough money to sponsor the content they want others to see. AKA big money advertising on the front page. Putting bitcoin as the centerpiece to reddit would kill it.
But with reddit, that's sort of the point. It's to see what the most popular thing is in each topic. What's popular, what's hot, what's new. When money gets involved, things get... weird. Things are done for profit, rather than creativity and entertainment. Some people don't care for karma in the current system, and just say what they think. Some want to get low karma just to see if they can. These will be ultimately eliminated with a money system in place. Dead subs will no longer be created unless the creator knows that people will go there. As I said, it'd be more circle-jerky than reddit is now.