My guess is simply that you have [assembly:CLSCompliant(true)]
in your assembly while MvvmCross doesn't have that set - see Base type is not CLS-compliant, what reasons of this warning?
If your project does care about interop with other languages and about strict CLS compliance, then there is a lot of info on it at http://msdn.microsoft.com/en-us/library/12a7a7h3(v=vs.110).aspx#Generics and you can find more about it here by including the C#
and cls-compliant
tag - e.g. https://stackoverflow.com/questions/tagged/cls-compliant