These are some examples (in order from most important/blocking to least blocking):

1 - full server side support for Outllook "complete" flag management on mails (To Do List from messages flagging)
2 - full server side support for Outlook messages colour categories
3 - support for "Recovery deleted Items" 
4 - synchronization of Rules between Outlook and the server (Exchange server-side rules)
5 - Public Folders

I will add more as they are reported...

Agreed.  Our Exchange users are heavily invested in Public folders, particularly for departmental calendars-- PTO schedules, duty rosters, etc.

I wanna bump this post and ask if something new is coming...

Is there any news on that?

We have wait many years, before we going into deeper test of SM (in 2020). We spent many time to bring them up linke as an exchange alternative, but we give it up, after some month. We take a look here and all weeks, the realaty comes back, that never a exchange alternative will be gone live and has near 90% of working. This not working, mapi not excellent working, calendar problems, group folder, and so on all time something not working....

Thats make me sad, but if we have a alternative, we changing the same day...

Hello Alex, 

We are sorry that you're having issues with MAPI. I will go ahead and open a ticket for you so we can look into your issues and resolve them. 

Please follow up with the ticket. 

Kind Regards, 

Alex, I had looked up your tickets and see that almost all tickets you had are about 2 years old.

I had Zach respond here and open a ticket to see what issues you might have.

We really have little to no known issues but do have features and functionality you might be waiting for in new releases?  We are here to help.

5 - Public Folders > also agreed, often used

Probably would add "delegated mailbox" but i gather there is something in progress already.