We chose .Net because we're both .Net developers (shocking decision). The idea behind NzbDrone has always been performance, when previously using SB (myself very little) performance was a big issue. Of course we knew using .Net was limiting, but that didn't stop us from using it, because we knew we could do what we wanted with the limited audience to start and expand to using mono if people wanted that.
Sounds like there are some people that would run it under mono, some people that will continue on Windows and some people that hate .Net so they don't care either way, to each his/her own. The argument that Linux is for servers is a little off base, IMO, I run a windows server at home and manage a large number of them in my day to day work, I really don't think one is better than the other for everything, each has its strengths and weaknesses and really is a Mac vs PC argument.
To those of you that are running NzbDrone - we hope you like it, to those that haven't tried it yet - please do
, to those that did and didn't like it - we would love your feedback and to those that never want to run it - that's okay too, we'll survive.
EDIT: Early post, sorry!