The single biggest threat to Bitcoin is governmental regulation. Personally I consider this the biggest risk - buying goods with Bitcoins avoids sales taxes (here in the UK - VAT, which is charged at 20% and a considerable income for the government) and you simply can't believe that any government is going to let this just happen...
I look forward to regulation - if you mean outlawing or general noise in parliament/congress. But if it's a scalpel attack (like funding sophisticated crackers, adding fear) then I think there is trouble on the horizon.
In the United States, I understand internet sales are generally already tax free. If for example, Europe made a lot of noise about tax evasion, that would get a lot of Americans curious. Since countries don't harmonize policy, later countries will take a trade advantage other earlier bans. Thomas Friedman was right about at least one thing. Countries are competing in a race to the bottom.
I think the single greatest threat is the ease with which a wallet can be stolen. I am already moving the majority of my bitcoins offline, will not put bitcoins on my phone, nor can recommend this to family and friends (no in fact I would now discourage most). That's hardly liquid. But by the time all bitcoins are mined (if even 2/3 are ever mined) then by that time all these problems would have been ironed out (or we wouldn't have gotten there). If bitcoins are still trading above $15 two years from now then why not for hundreds or thousands time more?
