Some of the missing calendar features we found post migration are:
1. You cannot add attachments to calendar invites like you can to an email. The attachment has to be on Google Drive and shared out to the guest list. Very inconvenient.
2. The Android mobile Gmail app does not allow Google Apps users to send accept/decline responses to meeting organizers that have Exchange/Outlook. So meeting organizers using Outlook do not know if can or cannot make the meeting. This basic functionality works for Google Apps user with Apple iPhones and blackberries. It is very odd that we cannot promote a full Google experience. We have to recommend iPhones and Blackberries to Google Apps users instead of Google Android. I am still waiting for a response >
https://groups.google.com/a/appsusersgroup.com/d/topic/northcentral/VLT4ybW6lVE/discussion
Not missing, but this tripped people up after coming from Exchange - they did not know how to "send" a calendar invite because in Google apps the only options are "Save" and "discard". They have to click "save" and then the user gets prompted to send an invite to guests.
One awesome feature that our version of Exchange did not have, but Google Apps did - blind copy calendar guests. No longer have to worry about meetings getting changed or replies to the whole group.