> Author here; I love the Mac App Store and want to eventually use MonoMac to port some of my Windows stuff over to it :)
I personally don't recommend this. I've been a big, big Mono booster over the years, but I have a hard time trusting the developers behind it. (For an illustrative example as to why, take a look at MonoTouch's problems with iOS 6. That bit my employer in the ass. It hasn't happened with MonoMac, but to me there's a "yet" involved.)
As un-sexy as it is, C or C++ seem to be the way to go. It's what I'm doing now, with a platform-appropriate hat on top that uses as little Objective-C (on OS X) as possible.
http://www.mono-project.com/MonoMac
I had A LOT of trouble finding publicly available data on how the Mac App Store is used. Sales figures, installs per day, price distribution, etc...
If you know something I don't, post it here and I'll do a follow-up.