Seems like a shame that it is bundled with the overhead of Office 365. Makes Slack feel like a lighter-weight option.
What overhead? If you don't want the other stuff don't use it.
In any case, this looks like a much better value proposition than Slack. However, I can test Slack in a second by going to their website. If I want to test MS Teams, I need to think about Office 365, sign up, find where to enable it, etc. Right now signing up for Slack is friction-less.