Have all collections implement generic collection interfaces
Many collections in the .NET framework that are not meant for general purpose usage (NameValueCollection, PropertyDescriptorCollection, ControlCollection, etc) do not implement a generic collection interface. So, in order to use LINQ to objects on a, for example ControlCollection, we must first apply the OfType<T>() or Cast<T>() operators, which wouldn't be required if this collection implemented IEnumerable<Control> or ICollection<Control>.
This is a sad thing indeed that a class such as StringCollection does not implement ICollection<String>.
I have little hope however, because it might be by design, because of compatibility concerns.