EDIT: To answer Hockey's question, I have highlighted the features that are complete in green. Events that are partially complete are in purple - also, because I can't portray this below, I will say that the interface for adding / editing events is like 80% complete.
Version 1:
Multiple Calendars - calendars can be restricted so that only certain users can see them
Recurring Events - events can recur in any way they can recur in the icalendar standard (RFC 2445)
Import / Export - anything that supports icalendar standard (RFC 2445). This will be implemented via qCal.
Multiple views of calendars - day, month, week, year, ALL events & event detail views
Printable Calendars - via css's print media
User permissions - like I said users can be given permission to add / edit / view any calendar or event
Javascript / Ajax - where it makes sense
Search-able
Custom fields on events - end-user can add "custom fields" to events such as "contact number" etc.
Ability to add events to multiple dates - I don't know that I've seen this done before. This will allow a user to enter an event, then click "add". Then it will take them to a calendar view where they select whatever dates they like. Has it been done?
Version 2:
Pluggable - the app will have a plugin system, although at first it won't be all that robust... probably similar (although more simple even) to wordpress for now, but the plugin system will get better
Theme-able - users can easily create "themes" without hackery
There is probably more... anyway... thanks in advance for your input.