I don't see the ability to use a slice as a dict key as particularly
more surprising than the ability to use ints as dict keys.  Someone who
doesn't understand how dicts work can use either of these features to
write broken programs.

I have thought about that example and it's precisely the kind of thing I
would like to work.  The behavior is consistent with that of using any
other immutable value as a key.  I don't have a use case right now (and
by admitting so may be dooming this change - but L. Peter Deutsch has
one, I think) but there's no way I would ever benefit from the current
behavior, whereas I _might_ be able to do something useful with the
proposed behavior.