I really like Teams. I do. But the current chat layout is not suited for professional chats. It's way to space consuming. It looks like something imported from MSN with smiley's that take half of the screen. I would say that Slack have nailed it because they focused on the ability keeping a tight window with lots of chat text visible. In Teams the window is full and starts scrolling after like 10 sentences.

Many people like to use English as their display language, but get confused with the US date format: 11/06 is October 6 in the US, but June 11 in many other countries. Why not display the date according to the setting in Windows?

Same goes for time, of course, where many use 24 hour format instead of am and pm.

make the left-hand navigation resizable

Please reopen the original "Timestamps for EVERYTHING"
https://microsoftteams.uservoice.com/forums/555103-public/suggestions/17691118-timestamps-for-everything

You have ignored request from the second paragraph
"Also team meetings should have this too", meaning timestamp, not just date.

Since Teams is a part of Office 365 why is there no App Launcher aka 'The waffle"in the top left corner ?

You should have a "what's changed" overlay everytime it updates, or at least a pinned topic on this page with a summary of the changelogs.
Given that it is a new product, some updates might include the missing feature that a team is waiting to switch over to Teams and it would be good to be able to discover this.

It would be great to have the Teams desktop app UI to be manually scalable. The default scaling should be set based on the system settings for scaling, but the UI should be scalable independent of this.

(should I mention Slack allows zoom in/out with Ctrl+ + and Ctrl+- key combinations and Ctrl+0 restores default setting)

I find the desktop UI too small for my eyes. I do have scaling enabled on my Windows computer, and other apps and programs work well, but the Teams client bothers me being too small.

When joining a meeting, the video and mic should be turned off by default and need to be turned on by the user either per session or globally via settings. This is the default behavior for both Skype for Business and Lync 2013 (as I recall).

Whenever I receive a notification and I click on it with the mouse, I then have to click a second time in the chat box before I can start typing. From usability perspective this is very annoying. Whenever you receive a notification from someone, and click on the notification bubble, you should be able to directly start responding to this person.

See thread for issues related to dark theme.

During testing of Teams app we found that users are quite often confused in regards to which team they are posting (specifically when they post to 'general' channel) - I think ability to assign different colors for different teams (by the owners/admins) would be helpful and could help to reduce amount of mistakes.

The Enter key opens a browser session outside Teams. Users expect Enter (specially in Web tabs) to execute the Enter command as expected in the context.

This will improve the usability of Teams due to the fact that users won't have to switch between teams and browser

Conversations in Groups starts at the top and forces old ones down the page. Conversations in Teams starts at the bottom and forces old ones up the page. Recommend you standardize on one method please. Confusing UX if you move between them.

would like to change the fonts

If there's only one more channel, it doesn't make sense to hide it behind a "More channels dropdown", you're not saving any space. I've noticed this with a few things in Teams, if there is room to display all of the channels vertically, or very few extra channels, it's bad UX to hide them behind a link.

Allow upload a picture to change the Channel header background. Will help users to differentiate between channels.

I'm member of several teams in Teams and when I switch between channels in different teams, other teams seemingly at random collapse the channel lists. This impedes my workflow when I participate in discussions in several channels in different teams at the same time.