I thought sure Bitcoin would be used in the sex slave and drug markets.
These two (and assassins-for-hire) are probably the use case where the governments would be accepting to throw the necessary resources to do the kind of big-data analysis necessary to track down the culprits.
(Follow the money trail. i.e.: follow the life of bitcoins along transactions, until a real-life event can be mapped to a transaction [e.g.: bitcoins were used to order some product online which was delivered at an adress. Or bitcoins were exchanged for cash at an exchange and were wired to a bank acount]. Do a huge amount of these trackings. After a while some pattern is going to emerge. This pattern might be used to get leads for real-world investigations).
Such tracking is well within the reach of various tree-letter agencies in the US (and in Russia, and in China, etc.)
Had not the founder of Silk Road been caught on some very stupid operational mistake, its likely that the US government would have gone this route to track him down (or it's still possible that they indeed tried the route, and on their way discovered a few operationnal mistakes, and decided to use those as evidence, in order not to admit their tracking capabilities)
Anonymity can be better achieved by what is kown as tumblers.
The cryptocurrency equivalent of money laundering.
You send bitcoins to a tumbler. These bitcoins are added to a big pool that is constantly mixed.
After a while, a similar amount of bitcoins (minus some fee) is sent out of random wallets from the mixing pool, to another address of you choosing.
Nothing is linking the 2 adresses.
If you try tracking the money (not easy because the tumbler itself is constantly mixing them) you see that the emerging BTCs come initially from a dozen of unrelated accounts.