Technically, any protocol if it's going to succeed is language agnostic. It'd be retarded to do a language specific protocol unless you're like Erlang and don't have to care about other languages as much.
The difference is Mongrel2 makes this assertion explicit and does everything it can to stay simple but still support languages you need to use. It does this by having a simple base protocol for doing fast async backends with 0MQ or HTTP, and by making the configuration system language agnostic as well.
That means if you run a Perl shop, then you can keep using your existing Perl HTTP setups with Mongrel2, write new stuff as 0MQ where needed, change parts to C++, and write configuration automation for your Mongrel2 clusters in Perl. Or any language. It doesn't care.
Compared to other servers, which still try to embed some specific language or write their own (Node, Varnish VCL), or have some odd config file only they understand, or force you to mod_* install languages, or use FastCGI in some bastard way, or hack up proxy configs to get page caching, ... this makes Mongrel2 much more agnostic.
But more importantly it comes down to the project's philosophy. I have a specific goal that no language will be king, so I'm going to architect the server so that nobody dominates. Same thing I did with Mongrel1 where I refused to let Rails dominate.