Default audio / video settings and remember user choices
There should be configurable defaults for audio and video devices. The defaults should exist at a global (admin) level, meeting level, and user level, with each successive level overriding the last.
Two examples of how this helps.
In Company A, almost everyone uses laptops for quick video calls. Defaults should be set for laptop audio and video on by default.
In Company B, most people prefer to have an audio bridge which they access via landlines or cell phones rather than laptop audio. Laptop audio (mic and speakers) should be off by default.
Bram Mertens commented
Too bad https://microsoftteams.uservoice.com/forums/555103-public/suggestions/17372452-have-video-off-by-default-in-a-team-call was rejected. I too would prefer to have video off by default.