vastly improve the process of inviting folks and managing calendar events. See Google calendar for reference.
Also, make it easier for standards-based calendar applications to talk to exchange servers. Right now, I use lightning in thunderbird and have to go through DAVMail, which is driving me nuts. I don't care for outlook desktop because I have to manage multiple emails across several providers.
Wade Dorrell commented
I've never had a good experience when receiving calendar invites from others that were sent to me out of Google Calendar. I accept using Outlook for Android's UI, including choosing the option to send reply to the organizer, and this does put the event on my outlook.com calendar, with all potential attendees listed but only me and the organizer marked as attending. But then the organizer later contacts me to ask if I'm attending or not because it seems this acceptance doesn't get reflected back on the Google Calendar itself, and indeed the list of people who are attending that are shown in Outlook never seems to update... I would have to log into the same-email-address ID Google Calendar and choose "Yes" from the "Going?" prompt to confirm myself, and the there I would see the other people who are going, and that Outlook has been wrong all along about attendance. I shouldn't have to log into Google, because I do not otherwise use Google for calendaring... that is the organizer's need, but it's not mine. (Although this failure does get me wondering if the grass is greener over there.)