One of the nice features on OSX with Address Book and iCal is that if I enter a person's birthday into their contact details it appears in iCal in the Birthday calendar. This has saved me on many occasions as birthdays are one thing I just never remember (along with other things like anniversaries).
I can't see a similar function in Zimbra but it would be really useful for me at least. Ideally there would be an 'events' area in the edit contact dialog where you could add a date and a description like 'Birthday' or 'Wedding Anniversary'. Once this was saved there would be a calendar called Contact Events that would pull these pieces of information out and associate it with the repsective contact. That way I could look at my calendar and easily send out happy birthday emails (or remember to buy flowers) without having to manually go through the calendar and add a series of recurring appointments.