Learn databases! Learn them, damn it! Learn how to design a database, understand why you should normalize your data and why you might break those rules. Understand why NULLs are a Bad Thing and why your DBA usually wants to shoot you for doing SELECT * FROM .... I am so tired of going into a database and seeing columns named "ingredient_1, ingredient_2 ...". I'm tired of fixing SQL injection attacks. Please, just learn databases, will ya!
There's a lot more about databases I could rant about, but I'll stop. Most serious developers today must work with databases of some sort or another and most suck at it. If you have a good handle on the basics, you'll trump most developers out there.