These won't stop bitcoin, but any one could definitely reverse Bitcoin's current direction:

 - FinCEN going after exchanges for not filing as money transmitters (and/or for not registering as money service businesses) for selling "prepaid access".  (This includes those exchanges serving customers in the U.S. regardless of where the exchange is located).
 - Coinbase, BitMe, BitFloor, and CampBX being served subpoenas to cough up the USD and BTC transaction history for each of its customer accounts.
 - Blockchain.info being raided for mixing service, logs sized
 - Update to Adobe Flash released in January includes previously undetected wallet-stealer code that executed on all systems at 8am UTC February 19, 2013. [this is a hypothetical threat.]
 - etc.
 - etc.
 - etc.