It may not have the high performance features of C or Java but its lightweight nature makes it easy to run in parallel. Making it well suited to run on a large array of cheap computers. And since horizontal scaling is generally better than vertical scaling, this is a big plus.
3. Something for everyone
4. Runs on the web (and everywhere else)
6. Ease of run
8. Open Standard
9. Easy to start
10. Gets shit done
Douglas Crockford has some good thoughts on the matter (video)