I wonder how difficult it would be to port C# to the jvm. It really is a better language. There are some parts of the standard library that are better in java, but overall I'd say C# is better there too.
Kotlin is nice too though.
I guess, you don't miss what you don't know.
Richer generics. Dynamic typing is there if you want it. Type inference is great, and then coupled with anonymous types allows for some nifty tool-supported functional-style pipelined programming. (RX and Linq). Upcoming language support for tuples looks pretty sweet too.
Type inference, C# properties (vs. getters/setters in Java), generics are way better, and of course Linq.
Java 8 makes this a bit better with lambdas but it's still a bit of a pain not having the above present. I still like Java though.