-Refactoring support isn't great. It's about on par with stock Visual Studio, but stock Visual Studio isn't good compared to VS+R# or IntelliJ. If you're writing "just" an iOS application, I can see this not being a huge deal, but my own use case is bringing over projects for OS X and iOS where I tend to have a lot of code and I'm very used to being able to do massive, contextual refactorings when needed. Can't do that in MonoDevelop.
-It's GTK#, so you have to accept that some stuff just doesn't feel native on a Mac. My mental map of keyboard behaviors no longer properly applies and it's jarring going between it and XCode or IDEA. It's gotten a lot better since I started using it on a Mac, to be sure, but it's not there yet. I compare it here to IDEA, which is also not Mac-native, but manages to feel a lot closer.
Now, one huge, huge plus, that I have to give them a ton of credit for: unlike Visual Studio, MonoDevelop is non-destructive toward project files. This is a power-user beef but a really important one once you get there; if you've hand-edited a csproj, Visual Studio will happily destroy all of your changes (it loads the csproj into an in-memory object model and writes it back out on save). This is infuriating when you have custom stuff in it - something as simple as a wildcard path in a file reference (which is totally fine as per MSBuild rules) will be mashed into a static list of all files at time of invocation. It is super bad, and if MonoDevelop improves some more I might switch to it, even on Windows, solely because of that lack of misfeature.
