1) The use of GIFs/memes. While this seems silly, it is actually a big deal because it lowers the formalization of communication. Having used this product, you just have much better free flowing conversation and many times these "fun" items help you get the message across significantly better (ex. asking for updates or bumping things can now be done in a funny way).
2) Creation of a horizontal matrix. Having tabs is awesome because it makes information more compartmentalized and helps keep the priority the same on each screen. This essentially adds an extra layer of depth into the product without overcomplicating things (and imagine one day having ability to live edit documents on a tab, while chatting and seeing metrics). Keeping a multi-dimensional communication design model is very scalable to mixed reality too.
Essentially I see MSFT Teams as "Slack meets Google Hangouts meets Email", and see it doing very well from a product sense. However, their business strategy of bundling with O365 will not help them hit SMB, which is a conscious decision I think they made. I would have actually used Teams as a trojan horse to capture stickiness and then upsell users to O365, but overall still excited to see where this goes.