What makes Mongrel2 special is how it satisfies these requests in a language agnostic andasynchronous way using a simple messaging protocol to talk to applications; not just serve files. Mongrel2 is also designed to be incredibly easy to automatically manage it as part of your infrastructure.
Other web servers do some of these things, but they either do them in a bastardized way or not all of them at once. Plenty of language specific web servers like Node.js and Jetty have asynchronous operation, but they’re not language agnostic1 . Other web servers will let you talk to any language as a backend, but they insist on using HTTP proxying or FastCGI, which is not friendly to asynchronous operations.