I think Rails is a good example of a great framework with the right language. Kind of the right place at the right time.
I came to Rails from Play framework(and also Node briefly) which actually got a lot of the Rails clone stuff right(they rarely ever do), and I was still stuck with Java(Play 1), and then later on Scala(Play 1 and 2) which turned out to be terrible.
Anyway, I'm not sure why people always spew FUD about the Ruby concurrency stuff. I guess the JRuby guys are doing a terrible marketing job. Charles Nutter has even admitted to that on Ruby Rogues if I'm not mistaken.
The process concurrency is fine for a lot, if not most, applications. Everything I've done at my job has been MRI except for one specific instance where I used JRuby. That wasn't because of concurrency though, it was just to talk to some existing Java stuff.
The only language I can see even peaking my interest is Go. Even then I've been so traumatized by doing Rest API's in static typed languages that I swore I'd never use them ever again. At the rate I'm seeing, get back to me with Gorilla in 3 years and Revel in 10 years. Granted Rails is not really the goal of these projects.