1. Fast compile times for most of development. I'm fine with waiting a while to do a special highly optimized deployment, but getting 95-100% of optimal performance with 0-3 second build times is really nice.
2. Controllable memory access through value types. Nothing getting in the way of C-like contiguous buffers and managing cache line or load alignment.
3. GC that gets out of the way. C# has a GC, but in most of my applications the GC never has to run because I rarely allocate GC-managed instances. It's definitely a nonidiomatic use of C#, but the fact that it's pretty still pretty easy to do is nice. And when I don't have to care about the GC's overhead, the presence of the GC just makes everything easier developmentally.
4. Ability to actually make use of the hardware. The compiler's improved massively in the last several years, and the way vectorization is exposed actually feels a lot nicer than my experiences in C++-land.
And less relevant to my own needs, but still interesting, nowadays you can directly link a C# library into a C/C++/Rust application just like any C library. Among other things.
Almost a decade ago, I was considering exiting the ecosystem in favor of C++/D/not-yet-even-1.0 Rust/etc, but the open source push and a sudden focus on performance basically made the jump unnecessary. C# occupies a really nice sweet spot.