This is a feature that is desperately needed! When we moved from our old CRM to Dynamics/Click Dimensions, we were shocked that we could not use marketing lists in email sends to EXCLUDE contacts. This is something we used all the time in our previous CRM.
It's quite often the case that we have a list of contacts that we want to send an email to but also a list we want to exclude from the same email. I know that you can manually remove contacts from static lists using advanced find, however this is extremely time consuming and also only works with static marketing lists. Most of our marketing lists are dynamic.
This is the point where you might say that we should create our dynamic lists to exclude the contacts we don't want, however this is not always possible. The advanced find is fairly simplistic and doesn't always allow us to generate the queries we want (for example you cannot have "or" nodes across entities, you cannot add formulae and you cannot create group/aggregate filters).
Exclusion lists in email sends as well as inclusion lists would be a welcome feature!