You can use Mono to program C# on Linux or Mac.

Yeah, but chances are your Linux or Mac users will already have python installed via their OS.

The biggest challenge for Windows users is remembering to go to python.org and not python.com to obtain their version.

And if you look at any of the udacity office-hours videos, when you see laptops, they're almost always Macs :-)

This is for people learning to program, remember? If they have to install Mono, that's just an extra hurdle, and doesn't get them any closer to their goal vs. just using Python.

Either way your going to have to install something, either a python runtime or a mono/.net runtime.

Plus your going to at least want to install a better text editor and probably set some environment variables.

The installation process for all of these things shouldn't be a big hurdle since all you'd have to do is follow a set of steps that can be easily demonstrated.