Stability
I've trialed Xamarin a few times over the years, and each time came away miserable.
The idea is genius... being able to write screens, call web services, use JSON.Net, etc using C# in an iOS app is fantastic. The problem is, Xamarin Studio for iOS just never stopped crashing every few minutes. I also tried using Xamarin from within Visual Studio on a Windows laptop, and connecting to my MacBook, but that was even worse.
In the end, Xamarin was never worth the time, money or risk, of developing our apps in, and I reluctantly had to travel back in time to the 1990s, and stick to using the appallingly-bad Xcode instead.
So, although this takeover is excellent news, I do wonder if Microsoft is finally going to make Xamarin's code stable, without it tarnishing Visual Studio's excellent reputation.