Off the top of my head the (very simplified) mechanics we regularly see are
- do not stand there
- stand there
- do not look at the thing (and very occasionally look at the thing)
- do something in X seconds (occasionally specified to "be moving")
- don't do anything in X seconds
- spread these markers out
- bring these markers together
- press the special button at the right time
- don't fall off the platform
- get hit by this aoe but not the other
- use the thing at the right time
- attack the thing now
- do not attack the thing right now
- move the thing there (then occasionally proceed to kill it on that spot)

I've no doubt that I forgot some mechanics that either crop up infrequently, are brute-forced or so ingrained that they don't register as a special mechanic anymore.