Missing .NET 4.5, which introduces C# 5 with async support. C# is a great language. Shame it's stuck on Windows and the "enterprisey" .NET framework.
Mono is a real alternative, many businesses run it production, including the likes of Fog Creek.
Enterprisey is a matter of perception and shouldn't be a slur.
I actually work at a .NET startup doing non-traditional things with .NET and we have the hardest time hiring new developers because of the "enterprisey" nature of .NET and the type of developer that generates.