It would be nice to have frozen Counters analogous to frozensets, so they are usable as dictionary keys.  One can of course create frozenset(counter.items()) but that means the set items are tuples rather than the original set elements, so it's no longer quick to check membership.  I can work around this in my immediate application but it seems like a shortcoming.  There are some other set operations that aren't supported by counters either, that would be nice if they are conceptually multisets.