I don't quite agree on the 'learn' part though, because it may sound frightening when for most purposes it is a direct C#+.Net port, so there's basically nothing to learn on the language/library front. It mostly revolves around the tooling (MonoDevelop is different from VS, mcs is different from the MS C# compiler, xbuild is different from msbuild, but source code is the same, assemblies are the same, sln/csproj files are the same). You will mostly lack WPF if you ever used it, and learn to use Path.Combine instead of hardcoded string concats and backslashes in paths (i.e write portable code), but you will benefit from having many Mono-only or Mono-bundled tools (csharp REPL[1}, ) and libs[0].
Miguel de Icaza regularly (re)tweets and promotes various community activities and achievements, so I recommend to follow him to get a feeling of what happens.
If you're daring enough, you can get on board and use emacs, which seems to be what the non-IDE Mono developer community revolves around, so emacs modes should be quite good (apparently, F# support is coming up nicely[2])
[1]: http://www.mono-project.com/CsharpRepl
[2]: https://twitter.com/simontcousins/status/293329223597297664