Couldn't you use Google Calendar for group calendaring? Just have everybody add the mygroup@gmail.com calendar as an overlay.
And folders are just a degenerate case of labels, where each message can only have one label. You can set up a filter to "label as emacs-devel and archive", and watch the unread count in the labels box. Just as easy as folders, though it's harder to have a "favorites" list like in Outlook.
Not exactly. Zimbra also keeps track of when people are busy and when rooms are available and things like that (in addition to supporting push email on iPhones).
As another commenter pointed out, it's more of an Exchange replacement than a Google Apps replacement.
And folders are just a degenerate case of labels, where each message can only have one label. You can set up a filter to "label as emacs-devel and archive", and watch the unread count in the labels box. Just as easy as folders, though it's harder to have a "favorites" list like in Outlook.