I see a lot of people who know three or four languages, but don't understand any of them really well or the underlying concepts. Or they'll focus over-heavily on the algorithmic side of the craft and neglect maintainability and readability. I see a lot of Ruby that looks like C.
When I finally do start branching out and learning new languages, it will be on a totally different level. I'll be diving into the entire ecosystem of the language and not just struggling at the surface.
But before that, I want to learn how Rails really works. I think I understand the difference between a library and an application, now I want to understand what a framework is and how one is built and maintained.