When composing emails, it would be very handy to have an "expand list" menu
item on the right-click menu of any contact list names you'd entered in the
to/cc/bcc fields.  Selecting this would replace the contact list name with
the individual names in the list, which you could then edit as normal.

This would be useful for those cases where you want to send emails to
everyone in a list save for one or two people, which is a fairly regular
occurrence for me at least.  (Especially as any changes you make in right
click->Edit Contact List don't currently seem to take effect, either just
for that email or permanently.)