Good parts: C#, and the entire .NET stack, completely skewers Ruby from a performance perspective, while not putting your coding in a straight jacket like Java does. You'll also find what in my opinion are vastly more powerful debugging and profiling tools, awesome threading support (be sure to check out both C# 4.5's async/await keywords and the existing Rx library at http://msdn.microsoft.com/en-us/data/gg577609), better versioning, better IDEs if you're into that kind of thing, and more.
Downsides: While this is gradually changing, I don't consider server-side Mono production-ready, so you'll be both developing and deploying on Windows. Deploying on Windows may prove expensive, but BizSpark (http://www.crunchbase.com/product/bizspark) will let you avoid that well past the point where you should either be successful or have VC, making it a non-issue. The community isn't as library-happy as the Ruby community, although that's definitely changing; one side-effect of that is that, where Microsoft anticipated your needs, life is beautiful, but where they haven't, things can get painful.
It depends, actually. For stuff I write in my spare time, it's actually perfectly easy to write C# in Emacs and drive the compile with a short waf script instead of a solution. You'll pick up at least some csproj files if you grab packages from NuGet, but it's not horrible to manipulate them in nxml-mode, and again, it's easy to keep all of your own stuff in simple waf files.
The downside of going this route, of course, is that using VS.NET or MonoDevelop can be a bit of a pain. When working on Kiln, I instead do most of the actual editing in Emacs, and use VS.NET as a solution manager/debugger/decompiler. Not ideal, but functional. You'd also have to go that route if you wanted to use Azure, I believe, since I seem to recall they want the solution file, not the assemblies.
Downsides: While this is gradually changing, I don't consider server-side Mono production-ready, so you'll be both developing and deploying on Windows. Deploying on Windows may prove expensive, but BizSpark (http://www.crunchbase.com/product/bizspark) will let you avoid that well past the point where you should either be successful or have VC, making it a non-issue. The community isn't as library-happy as the Ruby community, although that's definitely changing; one side-effect of that is that, where Microsoft anticipated your needs, life is beautiful, but where they haven't, things can get painful.
I'd see my previous answer to this question at http://news.ycombinator.com/item?id=3405362 too.