Covariance and contravarience is very useful, but it's quite annoying that it can only be used with interfaces and not classes/structs. I write lots of little monoid-like data wrappers like Named<T>, and I have to either go through the hassle of dealing with interfaces for something that shouldn't need it, or I have to abandon covariance. The .NET maintainers seem to think that nobody wants covariance on classes, but it would be very helpful to me.