It is lighter-weight and certainly doesn't have the giant development team or years of development time that went into Visual Studio, but that is actually a good thing. It does its job well and I don't feel anything is missing while I'm using it.
One of my favorite things about Xamarin Studio is that it is really fast! It takes a split second to load up the IDE and another split second to load my solutions, and in general it is always very snappy.
A great feature of the IDE that is easy to overlook is the universal search field. The omnibox in Google Chrome has raised the bar for usability and simplicity in search, and the universal search field in Xamarin Studio delivers a similar benefit. Searches span your entire solution and will even let you search nuget packages.
Lastly, a hidden gem I found recently is the built-in source code analysis. Under Preferences > Text Editor > Source Analysis you can enable a background process that will analyze any open files. This analysis can help you find issues in your code even before you compile. And importantly, turning this feature doesn't slow down the IDE at all!
To the Xamarin team I just want to say 'Keep up the great work!'