Full integration with Active Directory, for fine-grained permissions over all aspects of the mail/calendar system.
For example, with Exchange and AD, I can create a distribution group, and delegate "ownership" of that group to a specific user, so they can add/remove users to that group. I can set that group to "open" or "closed", meaning users can either join it/leave it without owner approval, or not.
I can give an arbitrary user access to another users entire mailbox, or give them only permission to "send as" a different user, or distribution group.
I can allow only certain users to send to specific addresses, meaning I can have a "My Entire Company" distribution group that only specific people can send mail to.
And then there are similar permissions/delegation options for calendars, and Public Folders, and even Skype for Business. If you have VoIP phone systems, and compatible phones, you can even access all of your mail/calendar/Skype messages from your phone.
I can set deletion and archive polices for each user, or a group of users. I can set mailbox size limits per user, or per group. I can create a "discovery search", meaning I can allow access to a user's mailbox, but only for mails that meet a specific search criterion.
And of course, there is a cottage industry of add-ons for Exchange to do a million other things. Mimecast, for example, allows automatic off-site archiving of all email (with an Outlook plugin to search the mail), and automatic failover to Mimecast's servers if Exchange goes offline.
It's just endless. Exchange has no real competition. Is it perfect? No. But it's better than anything else for corporate messaging, by a wide margin.