The biggest problem with bitcoin, and it hasn't been addressed yet, is that it is only accessible to the technically astute.
Aww, you think I'm technically astute.
You can't have a currency that isn't accessible to everyone. Money as we know it, is recognised and used by everyone. Its an easier way to trade than barter, which is why we use it. Bitcoin isn't.
I guess the Internet is doomed to be only used by nerds and geeks. No one could possibly operate it without technical expertise and... yeah. That argument totally worked in the 90's.
I'm sure Rushy will rush back saying how easy it is and that only a moron couldn't use it and blah blah, but its not as easy to use as cash. And there in, will lie its downfall.
It is just as easy to use, it's just that a lot of people have never used it before. Algebra is easy, but it feels pretty damn hard when you're in fifth grade. It just seems like a lot to learn, it really isn't.