When will we learn that money and systems (especially X new technology) aren't the issue (or the solution)? People are both. Until society admits both its faults and its failed solutions, it probably will be plagued with deplorable crimes such as those facilitated by Bitcoin (and USD and CAD and CHF, for that matter).
One of the commenters to the article mentioned, money was never designed to be a tracking system. That "feature" has only been added over the past few decades.
And law enforcement will always be inconvenienced in a free society, the kind our founding fathers most definitely envisioned here in the U.S. (and elsewhere, I'm sure).
In this case, since Zerocoin is actually a university-sponsored project (from the Johns Hopkins Information Securuty Institute), we can hope for a response... hopefully it will be a rebuttal from the researchers and not a kneejerk response from administrators.
Even beyond its obvious utility (for good or ill), zerocoin is a fascinating problem in cryptographic research, and we should be grateful it's occuring in the light of day and not as some super-secret project.
-Buggy Whip Manufacturers Association, circa 1890's
Far too risky for poachers and large-scale criminals.
In 30 years time, you consider retiring, and you have a significant portion of your savings in Zerocoin. Someone, somehow, manages to fraudulently acquire your Zerocoin savings, perhaps through a flaw in the Zerocoin system itself. You go to the authorities, and ask them to help you recover your stolen retirement savings. They reply, "Sorry, as you know, Zerocoin is completely anonymous. We can't help you, no-one can."
What do you do?
Digital currencies are the online equivalent of cash, not of a savings account or stocks. Like regular cash, it can be stolen (e.g., by hacking into your computer) or lost (hard drive crash with an outdated backup), and thus, you shouldn't have more on hand than you can afford to lose.
Different rules apply to speculators, of course, but you are talking about retirement savings. Putting your retirement savings in a Bitcoin wallet is about the same as having a pile of cash under your mattress.