Hold on to your butts...
1. Click the icon to open it. Flip to another app. Multiple times during startup it will steal focus as I'm trying to do other things.
2. Flip back to another app during #1? It'll sit at "Loading Teams..." for. F'ing. ever. You won't get any notifications during this phase. You will not show up as online.
3. When it does load, I'm always 2 months back in chat conversation and have to spend 2 minutes scrolling down to the current time to see if there's anything new.
4. There is no way to skip to latest...
5. Notifications don't integrate with anything, they are custom. They are quirky and peculiar. There have been times that _none of them_ come through - no badge, no notification, no mobile push notification, no nothing - if I hadn't flipped to the app I would have no idea I got any messages at all. And yes, these are direct messages that I have notifications turned on for.
6. Uploading files is annoying as F. Drag and drop into the message. I have to click into the message input box to even start adding any other text (seems like an obvious transition to me)... when it finishes uploading you might get kicked back out of the text box because it wants to ask if this is the same file as you uploaded before.
7. When focusing on the app I'm never, read _never_, placed into the input box, even if I just saw a notification (miraculously!) that I'm trying to reply to. (Skype also has this same problem.)
8. Defaults to emailing you _for every F'ing thing under the sun_. It's an F'ing chat app, why are "you're missing the conversation!" emails the default?
9. Navigation is a clusterF. Three layers of vertical panes of navigation (ala Slack). Then add on top of that horizontal navigation _in every single one_. And not just a fixed number of tabs as navigation, you never have any idea what it's going to look like because...
10. "Integrations" is just BS. All they mean in Teams is a new tab you can add to the horizontal navigation in #9.
11. Any of their logic regarding recent conversations, recent groups, etc. and what should be displayed in the left navigation is crap. I've only got one team that I use, but I have to expand that single team to get the other "chats" in it? You've got thousands of pixels of vertical real estate here, why do I have to constantly expand the _one thing_ in the list?
12. The "wiki" has no navigation by default, you have to try and figure out (amongst the 15 different layers of navigation) where to expand to find the pages in it. Hardly a wiki, and even the editing tools are quirky and annoying - things disappear and reappear at random.
And that's just off the top of my head. This is seriously the _worst_ chat app I have ever used in my life. Some random team at MS took Skype and decided to take the worst UX features of it and make a Slack clone. The fact that they started pushing this to businesses as part of 365 should involve mass firings.
Edit: Formatting and a word... and a number.