Firstly zimbra has an amazing mail filter.
The best and the most important part of it is due to the "Named Header" option that allows you to set your own custom header filter. This is something nearly every filter miss out. You have to edit the code and add new headers if you need them.
What i would to have in the filter is to be able to select Read and Unread messages.
For example, i want to create a new Tag that displays all Unread emails.
For me that would complete the features of the filter.
I can't think of anything else that it doesn't have