Given that money laundering and drug transactions have become major parts of bitcoin's current use case, law enforcement's tendency to seize bitcoins puts yet another deflationary pressure on the system. The biggest obstacle to bitcoin becoming a currency rather than a speculator's darling is the stability of its value. Meanwhile, the system's incentives seem to promote hoarding in hopes of a price spike.
Not that this is problem in and of itself. Bitcoin's decent anonymity creates very real value, and it looks poised to be a medium of exchange. It just puts a damper on the hopes that future transactions will be denominated in bitcoin rather than dollars.