Rapid prototyping. I'm not talking entirely out of my behind here; I've done ruby before. It's a joy to write, especially compared to something like Java. Unfortunately, that convenience and syntactic sugar comes at a price - performance.

Ruby is like a back-of-the-napking sketch, almost: it takes nothing to get started, you can just jump in and start writing. Other languages are closer to gradations of a proper engineering diagram, the sort of thing you draw up once you've fleshed out your idea a little and want to scale.

As much as I hate bothering with a proper diagram, it's very hard to scale based on napkin sketches.