What about crypto currency makes it hard to regulate? The US Gov regulates all financial transactions, including face-to-face exchanges of cash, which is just as untraceable as crypto currency unless you have the budget to follow everyone around in spy vans (which the IRS does not).
2. Unless you coordinate internationally, the nations with less regulation will launch successful businesses on it and dominate the market - imagine US having some crazy internet regulations in 1996, and other nations not having them. We'd be having this discussion now not on Hacker News, but on Новости хакера, or on 해커 뉴스. And US economy would be a few (dosen?) trillion dollars poorer.
I think the founders of e-gold and LibertyReserve can tell you all about how hard it is for the US to regulate financial services conducted over the Internet using databases and tokens but over the planet by actual people and corporations.
Apples and oranges. You can't sieze the server or domain name and turn off cryptocurrency. That guarantees that the back end keeps running. Then your only way is to regulate the usage of it, but when that can be obfuscated with things like tor, VPN, public WiFi, etc it's going to be incredibly difficult to do any sort of meaningful enforcement. Add decentralized 'smart contracts' such as what ethereum has and it can be run beginning to end in a fully decentralized way.