Fix these missing features and bugs, and Teams will be tolerable
Allow users to hide any UI element, especially the completely redundant "Activity" feed but also Calls, Meetings, etc. If it's not relevant to a user's work, they shouldn't have to see it.
Get rid of mandatory "General" channel. This should be a toggle, with default "off." Let teams create their own channel selection as their usage needs dictate.
Channel topics need to exist. Channel topics have been a thing since IRC was invented, and literally every chat system since has had them. Why? Because they're useful. So add them.
Add /me actions. IRC has it, Slack has it, Hipchat had it... people use it.
Improve threaded Reply so it's visually distinct from "new conversation" and most importantly make sure it's ALWAYS visible, as sometimes a conversation won't scroll completely, leaving users with the mistaken impression that they're replying to a conversation rather than starting a new one.
On the topic of threaded conversations, make this optional per channel. Just like the mandatory General channel, forcing things down the throat of users gives us a bad impression of the product.
Performance of conversations being marked as read is uneven. Sometimes a conversation will mark read immediately, other times it's necessary to toggle between two channels to get the conversation un-bolded.
Focus on the user. Looking at the Teams roadmap, it's all about pleasing the bosses and absolutely nothing indicating the Teams dev, ahem, team, cares a single whit about UX. The result is a terrible user experience and a bunch of people in the C suites geeking out about features nobody but C-levels want. This is really Ballmer-era thinking, Microsoft. Focus on the user for once.
Oh also, whoever came up with the default set of emoticons needs to be taken out to the woodshed. The ones that aren't downright creepy are still animated, and most of them are creepy as all getout. Horrid, really.