Node leverages Javascript's first class functions to allow you to program the server in a dynamic scripting language while getting very competitive performance.
Node isn't as fast as Haskell, Erlang or Go. But it is competitive with Java, and it outperforms Ruby, Python and PHP.
Haskell, Erlang, Go, Java, Ruby, and Python all have evented IO webframeworks, but they also have blocking libraries to serve as pitfalls.
Despite it's warts, Javascript is the lingua franca of the web and since browsers are evented, not only is Javascript built for evented style programming, most web developers are used to writing evented Javascript.
Also check out this register article: http://www.theregister.co.uk/2011/03/01/the_rise_and_rise_of_node_dot_js/