Scheduling a meeting is a pain. Finding free spots on people's calendars can be difficult especially with a distributed team that might span a couple of timezones. Another wrinkle is being able to combine work and personal calendars to avoid needing to try and keep things like dentist appointments in sync across both. I've yet to find a calendaring app that deals particularly well with these sorts of real-world problems.
Having the ability to "share, but not co-mingle" work and personal calendars would be great. By default personal appointments would be private but visible to the owner, and editing in either the work or private calendar would sync (across systems and domains - don't expect people to do everything in just Google or Exchange). Only personal appointment specifically marked as 'blocking' would show as unavailable during work hours (but when viewing a calendar request I'd see any work or personal appointments that might conflict).
I have an added level of complexity for scheduling though. I travel quite a lot, and often in very different timezones so scheduling a meeting (or letting other people book my time) can be complicated. It shouldn't be, computers really should be able to help us with things like this.
If my travel plans are available, there is a way to find out when I'm on flights (and Outlook does a pretty good job of understanding the reservation emails and blocking out my calendar) so it knows where I'll be... why not adjust my 'work hours' to local time and make that availability visible to people trying to schedule appointments to avoid the need for me to reply and try and reschedule (often multiple people).
Calendaring is such a fundamental part of everyones lives, it seems strange that despite lots of pretty UI changes and the ability to import holidays the capabilities haven't really improved that much since the tools I was using in 1999 to todays systems.