[Recurecur]

I'm aware of Mono, but it's essentially a second-tier implementation. .Net on Windows, and the JVM on Windows/Linux/Mac enjoy support from large organizations with lots of resources. How promptly will Mono security holes get fixed for instance? How much scrutiny is there for them in the first place?

Also, the JVM beats the CLR for most benchmarks I've seen, especially real-world benchmarks. Perhaps with Microsoft open-sourcing the CLR runtime, Mono is as competitive under Linux now. That said, the JVM has scaled onto massive systems - I'm skeptical that Mono has similar capability.