Also I'd like to see more detail on why exactly writing custom servers is so profoundly important all the sudden. I hardly see the advantage of even nginx over apache, though that's a different issue.
WebSockets probably. None of the traditional server-side web frameworks (Ruby on Rails, Django, Java Servlets, ...) can deal with them because you need to maintain open connections with each client. That also makes asynchronous I/O important (a feature of nginx over apache).
Most WebSockets solutions are clumsy at best, because you need to run something independent from the rest of your web stack. Unless you use solutions like Wt (http://www.webtoolkit.eu/wt).
What do you mean when you say "independent of your stack"?
memcached, postgres, rabbit, etc aren't written in the language I use, but they're very much a part of my stack.
Using an external application to hold the WebSocket connections and be the middleman between your app code and the user is not automatically clumsy. Clumsy would be re-writing your app in JavaScript because you believe it's the only language suited to WebSockets.
Also I'd like to see more detail on why exactly writing custom servers is so profoundly important all the sudden. I hardly see the advantage of even nginx over apache, though that's a different issue.