Everything, fuck microsoft.

I have two instances open for two different inboxes and try to pull one up on the taskbar, the wrong instance will pull up every single time without fail.

That sounds like how everyone always guesses wrong which way to plug in USB, not a specific problem with outlook.

Labels should be the default but UI designers on all platforms only care about appearance. Googling says you can change the taskbar to not combine instances and show labels. That way you always know what you are clicking.

On the new Outlook, clicking on emails that pop up opens them... in the background. Insane design choice

For some reason, opening attachments is a lot more painful that it needs to be in open emails.

Maybe there is an option to remove "open file in browser" but if there is I haven't found it. Awful, awful design

Jesus. Basically everything about it. We use the web client at work, so might be a different set of annoyances.
deep breath I use slashes in 70% of the e-mails I write as the punctuation mark that they are - I have never and will never want it to start randomly guessing what files I want to attach to the e-mail when I type a slash.
I actually didn't hate that if you typed something like "23rd" it would make the rd super-script - until the day it started doing it so incredibly wrong that it would super-script the '3' and the 'r' and change the 'd' to a 'th' so it read "2^3r^th".
Several times per day it will show a folder name in bold with a number in parenthesis telling me I have new messages in that folder, but when I click on the folder it highlights nothing as new messages, so I have to just remember what the last message I actually read was. Search sucks. Rarely can I open attachments. I told a coworker just yesterday that if they want to keep something secret from me, their best bet is to put the secret in an e-mail attachment and e-mail it to me.

My inbox view gets messed up every now and then and I have to keep resetting it.

Honestly what doesn't annoy me? The list is pretty short. I've never used Outlook before my work, I'm used to Gmail. Outlook feels remarkably shit for something so commonly used. Microsoft ia horrible at UI design imo. I can never find what I need and things that are supposed to be simple are somehow very convoluted.

You can't AND/OR combine filters. If i want to move stuff into "Gitlab trash" automatically, I need to use different rules instead of one with multiple applying conditions

That it will automatically try to disable com add-ons even if you tell it not to or create a group policy that tells it not to disable certain add-ons. Still on some machines they will disable. Disabling and re-enabling them from the list often won't fix the issue as you have to first re-enable them in another menu. If neither work then you have to completely re-install the add-on.

honestly their office suite is something I feel they did overall pretty well with. Granted plenty of changes and little things that aggravate me but hard to say as Im not actively using it. Outlook and excel in particular are standouts.

Its popularity.

Trying to find the search button, depending on what you have open it is hidden completely.

The minimize button closes the application.

That I cannot export mailboxes to other clients. Screw .ost !

When you try to change views in a shared calendar, and it automatically switches to your personal calendar in the other view, causing you to go back to the first view, change calendars, then do the exact same thing again.

Every. Damn. Time.

Also cached exchange mode is a disaster...

Idk why MS lets you use add-ins but then they go and disable it if it starts misbehaving according to Microsoft's determination of it doing so and not the actual add-in causing problems. OH—and then guess what? You can't fucking enable every add-in. For some, you can only tell Outlook to not disable it for a month and then it's the same thing all over again. Fucking Microsoft!

EDIT:

Also, who is the bird brain that decided it was a good idea to force add-ins to be managed partially via the browser? Like if I click one of the manage Add-in buttons in the Outlook app, it works just in Outlook but the other one opens up browser.

Also also, why tf are there 2 different Outlook apps now? Why should an image have Outlook New and Outlook Classic? Why tf does the mail app even exist at this point?

I started using Outlook365 in browser during the pandemic, and never switched back. Not that much better, but I prefer the web app running in Firefox to the actual program.

Plus, at this point, if I even opened Outlook proper, Windows would probably freeze while trying to index all of the emails that have come and gone since the last time I opened it.