Examples at the ergonomic level: wrists have 1 degree of freedom, while elbows have 2, and the neck has 3; the human color space is 3-dimensional; the QWERTY keyboard was intentionally designed suboptimal, to avoid key jams in mechanical typewriters.
Other notes: HCI largely concerns individual psychology, while CSCW tries to reach toward group psychology and sociology. Ethnomethodology can be considered to generalize subjects like ethnomusicology, ethnobotany and ethnomedicine.
There was an interesting discussion of how several recent high-tech user interfaces seemed to ignore the social dimension at some cost to their effectiveness. Examples included Andersen's Viking museum, Bill Gates home music and LCD art displays, stockbrokerage support systems, and even an SF novel (see first item of class discussion).