Why does this simple .NET console app have so many threads?

Try running it outside the debugger (i.e. press Ctrl+F5 instead of F5). You should only see three threads - the main thread, the GC thread & the finalizer thread IIRC. The other threads you see are debugger-related threads.

Project + Properties, Debugging, untick "Enable the Visual Studio hosting process". I can't discover what it is doing. As soon as I tick the "Enabled unmanaged code debugging" option to try to get a peek at these threads, they no longer get started. No clue. But I'm sure it's for our benefit :)

If you run it without a debugger attached, there are significantly fewer threads. And those would presumably be the finalizer thread, and other house-keeping CLR stuff.