David Berlind has a list. Among them:
How many times have you received an appointment request from someone who doesn’t share the same e-mail system? How many times have you had to cut and paste a bazillion times from the e-mail message to your calendar?
Are your appointments replicated to some place other than your primary system? A system at home for example? A PDA? A Smart phone? Having my calendar available to me (and others) at any time and in any place is a huge advance. But here’s the rub. I’ll be in a meeting, and my calendar pops up a reminder. When I dismiss the reminder, why isn’t it then wiped out in all the other places where my calendar is replicated?