1. Security of value: A continuos 0.000000001% attack from Central Bankers debasing currency.
2. Government Laws and Regulations: Used more for drugs, porn etc than any other currency in the history of the world so far in violation of the law the world over.
3. Other government's printing their own fiat currencies. If fiat really catches on, we will see other countries like the EU, China, Japan etc trying to print their own money. (Oh wait, I guess it has caught on already).
:-)
1) Government regulations, Taxation, centralizing, or blantly outlawing bitcoin
2) Government digital currencies, if bitcoins technology really catches on will we see governments forming their own regulated digital currencies?
3) Security. 51% attacks and Quantum Computing (I understand SHA256). Quantum is still decades a head of us.