I really like the "anonymous currency" sales aspect that helped get Bitcoin adopted. At best, Bitcoin gives its users security through obscurity - people don't understand how it works, so they think it's untraceable. The very nature of digital currency - especially the no double-spend provisions - make it traceable by design, far more so than cash, precious metals, and even unmarked gemstones. Due to their long history, the traditional currencies have better developed traceability schemes, but when someone really puts their mind to developing the methodology (and really, this is just a matter of convincing the courts that the method constitutes proof beyond a reasonable doubt), tracking down and proving the identities behind bitcoin transactions will be more forensically certain than fingerprinting or DNA evidence.
And, yes, there are some techniques that users can try to use to try to further obfuscate their identity, but the fact remains, the blockchain is publically available, every transaction ever made is recorded, decipherable, and you can bet already archived by law enforcement.