Well, if you haven't experienced it already, you can essentially write your own cookie-cutter Web 2.0 app (full of all the AJAXy community goodness) more robustly and in much less time (and lines of code) than with any other language or framework.
That is, any database-driven application that you want to fill with all the hottest plugins like Google Maps API and Flickr and such can be done relatively easily. People are busting out with new plugins for Rails everyday. Plus, it really is a joy to code in Ruby with the Rails framework; you'll think to yourself, "why didn't someone come up with years ago? It would have saved me a lot of time."