We stopped using Mono after a few months. With .NET's Task Parallel Libraries we saw an enormous performance difference between Mono/Redhat vs. .NET/Windows, with the latter often running many times faster. Tuning the GC and heap size didn't make much of a difference.

.NET seems to be well worth the cost of running Windows, but the consequent platform and vendor lock-in is still a concern for us.

Mono's parallel implementation is really bad, yes. I've had similar problems with it (despite the fact that the Mono SGen GC is usually pretty decent at keeping up)

Maybe you should contribute a better one. ;)

That's an optimistic world view.

Getting a web app built on .NET/MVC4/IIS running on mono isn't a trivial endeavor, and the open source implementations rightfully tend to lag behind the closed implementations.

What you're saying is only true for perhaps the simplest "Hello World!"-level apps. Much beyond that and it just isn't trivial to use those alternative tools effectively, even when you're specifically targeting them and not actual .NET.

I've deployed fairly large .NET applications to Mono with only trivial modifications. They behaved equivalently, even if Mono's garbage collector isn't as good.

My smaller experience only had problems within SqlDbType particularly with Date acting as strings and Structured not existing.