SQL is easily the best option at the moment, but unlike the experimentation in programming languages that we've seen in the last 10 years or so, we've yet to see much experimentation in query languages (as far as I know!)

ReQL, what RethinkDB uses, was an interesting direction because it allows people to start reasoning about a particular query. This, IMO, is the biggest thing that SQL lacks - the ability to reason about a query. I'd love to see a way to extend SQL that solves this.

The other thing that really, really sucks is that SQL has some terrible documentation and tutorials. Even for PG, for example, it just mentally takes a long time to parse through the documentation to understand how to use a function that solves your use case. This might just be a lack of Stack Overflow posts out there, but it's definitely not as friendly as, say, finding answers for Ruby code.