You can get very far with Ruby or Python before having to learn a bit of C for extensions. I've been using Ruby for years and I've yet to run into a situation where I need it, though admittedly I stick to web stuff.
Don't get me wrong, I'm a big fan of high-level programming languages. But things like threading, graphical interfaces, multimedia, embedded programming, binary distribution etc. just isn't very well supported. Partly because of technical challenges, but mostly because there isn't a critical mass of people doing those things.