The problem with BitCoin is it is nowhere near as anonymous as people think it is. In fact, it is even less anonymous than current currencies.
Consider the dollar. If I take a dollar in cash and deposit it in the bank, and transfer it to you, and you take it out of the bank - that dollar is now different. Those two dollars have different serial numbers. This is because in the eyes of the government, the law, and everyone under the sun, all individual dollars are the same and interchangeable - my dollar is as good as your dollar. This is what makes money laundering possible and why governments have a hard time battling it - it is pretty easy to funnel money from crime into another medium / person and "wash" the money in a way that makes it totally impossible to tie it to a specific crime, because all dollars are the same.
BitCoin is not like this. A bitcoin is a unique value and as it is passed from one wallet to another, that transaction is logged throughout the network. For any given bitcoin, you can trace the path of THAT SPECIFIC COIN from the time it was created to where it was today - seeing all of the wallets it passed through and what IP address owned that wallet at the time. All law enforcement needs to do to tie a specific bitcoin to a specific individual, for the purposes of an investigation, is to tie a wallet ID to an individual. Thus, any bitcoins used during the process of ANY CRIME are subject to seizure! I have never had anyone explain to me how to get around this problem with BitCoin. People have weird pseudo-anonymous hacks like "use ToR" or "Use a VPN", but all these things do is make it HARDER to tie an individual to a wallet, it is not impossible. In fact with the proper warrants and wire-taps it is trivial to tie a wallet to an individual.