IIS also has too many knobs that can get fiddled and frig things up. Before we switched, I'd always be on support calls to help a customer fix one of our products after their sysadmin tweaked something in a way to break us, or installed some other piece of software that stomped on us.
IIS also has a very conservative set of defaults if you naively install it. Seriously, I've seen a lot of instances where customers install IIS, and CSS doesn't get served - you have to install an additional Static Content module. People don't read instructions, or bother to run required setup scripts. So you have to do a bunch of additional work with less-than-friendly installer authoring programs to make sure that features that any self-respecting web server ought to have out of the box are available.
ASP.NET 5 is essentially based on OWIN as a pipeline which uses a pipeline of async Func calls (that can be wrapped up into "middleware" components). NancyFX, actually implements an OWIN middleware which I think would be easy to tie into ASP.NET 5, but I haven't tried yet.