The ability to share folders, calendars, and address books amongst users, as well as the ability to simulate a Microsoft Exchange server for Windows Outlook users.
It can be difficult to administer. The backend is fairly complex, but it is getting better. I don't think you need to be an expert to administer it, but you probably should be. The documentation isn't always that great, but the community support usually makes up for it.
Zimbra collaboration is primarily a backend, not a front-end, although they have a pretty decent webmail client.. You can use pretty much any email, calendar, or address book client that supports standard IMAP, caldav and carddav. There is a community version and a pay-for network version, where you pay per user. We use the Network Edition, so I can't comment on the community version.
We are very happy with the reliability and support of the product. Its rare that we run into a problem we can't solve with community support, but phone support is not always the highest quality. You would use the Network Edition for the added features, not for the phone support.
It uses Postfix as a backend, which I would recommend that you are familiar with before using. If you don't understand how the postfix system works, you will have trouble managing Zimbra. This is not a system for novices.
Email server administration is much easier with Zimbra. It allows us to manage our email server with a number of open-source technologies, including built-in anti-spam, shared calendars, shared address books. We've definitely realized significant savings in the time required to administer IMAP, webmail, etc.