For the last month or so, I've been spending some time on a C# application to perform vulnerability analysis of PHP and ASP code. The language was imposed on me by external constraints, but it turns out to be a very reasonable choice for this sort of problem. I'm not doing anything particularly groundbreaking or clever, so it ends up that having really good libraries and a reasonably expressive language is more useful, in terms of immediate productivity, than having a really powerful language and half-assed library support (see: Common Lisp).
I'm sure there are people who scoff at properties as being purely syntactical sugar, but I found judicious use of them can really improve readability without introducing any new dangers to class invariance properties. I was recently tried (again) to tackle a design problem that I've been struggling with in Botan for some time, and realized that properties would be very useful for me. Unfortunately, it doesn't seem possible to emulate them in C++ without paying severe penalties in source code complexity (writing a lot of boilerplate code, or using macros to generate said boilerplate).
The standard library seems fairly good, but is also deeply limited in much the same way that the Java class library is. The support for XML processing, GUI programming, and so forth has all been fairly pleasant and easy to learn. But at the same time, obviously useful utilities such as option parsing and filename wildcarding are nonexistent. You can find several implementations floating around for both of these, most of which are limited, buggy, or just plain bad - one would think that would be hint enough that it should be built in. It's as if nobody at Microsoft ever wrote a nontrivial command-line application in C#.
And would it kill anyone to include parser and lexer generators in the toolset? It's rather sad that in many ways the best language support environment is still for C. While searching for one I could use, I checked the Mono code - and found that it generates the C# parser with a parser generator from the late 80s written in K&R C. Go figure. I ended up using CsLex, which despite not being updated for over six years turned out to work fairly well.
Nits aside, I'd say I'm overall fairly happy with C# as a development environment. Without a doubt there are problems for which it is totally inappropriate, but I can't imagine that I'll ever write Java again (not that I ever wrote much code in Java, anyway). And while Mono's current (beta) implementation of Windows.Forms is horrifically buggy, slow, and visually ugly, the GTK bindings mean that I'll probably never follow through with my original plan of learning GTKmm.