Well, as long as baseClass also implements IComparable, then it mainly
depends on how you later use it. For example, if you have a
List<subClass> and call Sort(), it will use Comparer<T>.Default; if
subClass doesn't implement IComparable<subClass>, then this will fall
back to using the non-generic IComparable interface, so as long as
baseClass : IComparable, things should still work. Since we are
talking about a class (not a struct) there is no boxing overhead here
- just a little casting overhead.

Of course, if you only use comparers based on baseClass (i.e.
List<baseClass>.Sort()) then it will use your IComparable<baseClass>,
and no casting is required. There is nothing to stop your subClass
implementing IComparable<subClass>, but it gets confusing - and you'd
still want the comparisons to work out the same regardless of whether
IComparable, IComparable<baseClass> or IComparable<subClass> was used
- so perhaps just draw the line at IComparable and
IComparable<baseClass> (both calling the same actual method to do the
work).

Marc