The big real-world implication I'm aware of is that law enforcement can match DNA they found somewhere against 23andme's database. Then if you (or any of your relatives!) are in the database because they've ever used 23andme, they'll find that out, and they can use it to investigate or prosecute you.
Whether you think that's a good or a bad thing depends a lot on whether you think the cops should be able to succeed if they get a hold of someone's DNA and are looking for the person to match their sample against... that success is, to me, much more likely to be a good thing than a problem, but that may not be the consensus view here and it's certainly a massive, massive privacy implication.