like kevin said, the most efficient way to make enhancement requests is in bugzilla. if you can't find an existing entry go ahead and file a new one.
to address some of glenn's comments:
1. select more than one attachment at a time
this can certainly be tedious. you can, however, select all the files and "send" them to a zip archive and only attach a single file. i think we have a couple bugs open on this.
2. turn off the reading pane
http://bugzilla.zimbra.com/show_bug.cgi?id=6221
Bug 6221 - Reading pane On/Off Preference or Sticky
3. appointment to PM default the end time to PM also
good for bugzilla. also try dragging new appointments.
4. Appointment repeat should allow you to choose days such as every Monday, Wednesday, and Friday. Right now you have to create a separate appointment on each of those days.
in the "QuickAdd" dialog, the "Repeat" field has the choice "Every Week". if you click "More Details..." and click the "Every Week (No end date)" link, you get a "Custom Repeat" dialog where you can select on which day(s) of the week the appointment will repeat, set an end date, etc.
5. click TO:...have the global address list or local contacts
we're working on some ways to make contacts more accessible, for example, gal-access for autocompletion. if there is a specific solution you'd like to see, definitely post it in bugzilla.
http://bugzilla.zimbra.com/show_bug.cgi?id=5893
Bug 5893 - autocompletion should access the GAL