Distribution List modification is impossible for owners
Owners of distribution lists used to be able to modify the membership directly through Outlook. Since the move to O365, they can no longer do this and must contact an administrator to make these changes. It would be more efficient if the DL owners could change the lists they own through Outlook
John L. commented
Azure AD Sync write back for distribution groups is a must. We lost this functionality for users when we moved to O365 using Azure AD Sync. Write back for mail enabled security groups is possible but only if you license for Azure AD premium. A basic functionality that existed in Exchange for ages is now mostly unavailable in O365.