Some of the comments in that thread illustrate a widespread misconception that C# isn't cross-platform. We're finishing up a game that runs on windows, osx, ios, and android, with a server on linux; and it's written in C#.
Microsoft's CLR and BCL implementation aren't cross-platform, but I haven't used those in quite some time.
Java, on the other hand, is, for better or worse, used heavily on all the different platforms. Moreover, most Java libraries (like Swing. Does anybody still use Swing?) are cross-platform.
I find F# a much more interesting language than C#. Do you know how F# behaves on Linux? While I've heard of people using C# on Linux and Android, I haven't heard anything about F#. Then again, the Linux people might just prefer OCaml.
WPF was a definite non-starter for mono. Just too huge of an undertaking; much more focus on GTK#. Sometimes it's easier to bring something else to Windows than to bring Windows to everything else. In the case of WPF, it seems like not bothering was probably the correct move...
My understanding is that F# works just fine on linux and mac, but I haven't had the opportunity yet. Quite interested in this as well.
Something that seems to be a disadvantage with Java over native builds is the way it complicates the task flow for users trying to install your software. Either they have to download a large distribution that includes the JVM inside it (are distributors even allowed to redistribute Java like this?) or else they have to download and install the JVM, and then configure their new application to point to it.
How does this work when you're using c#?
On desktop, the necessary parts of the mono runtime would just be included with the distribution package; much like any other DLL. I don't think any developer in their right mind would ask people to download the mono installer. On the plus side, this doesn't pollute their system with registry entries or system files.
It's clearly not optimal to have that overhead, but it was preferential to porting to each platform in this case.
How about your server code? Is that ASP.NET running on Linux? Or something else?
On the server side, we just build the latest stable mono release and we made a socket server wrapped around libev (similar to what Manos de Mono does).
(I guess I lied, we have a small amount of web stuff in Ruby, but mostly just to serve content.)