It's got a non-intimidating C-like syntax, has some powerful features (e.g. being able to use anonymous functions as closures), is garbage collected, and can run client side. And you've got three or four well-funded organizations full of smart people duking it out over who can make the fastest implementation.

The syntax is a bit ugly if you're used to something like Ruby. Comparing Express to Sinatra, I miss the block syntax and the implicit returns (although JS handles passing in multiple blocks quite elegantly).

Maybe what we'll see are a number of languages that compile down to JavaScript, sort of RJS on steroids.