I quite agree with DOnald Simanek, but there is a crucial point that
must not be forgotten.
There is tremendous pressure throughout the system, of which the student
evaluations are but one symptom, on professor never to ask students to
do things which they find "too hard". I and many colleagues have gotten
into trouble for doing exactly what you suggest. Even tenured people
often cannot buck a tide of relentless mediocrity supported by
administrations who care more about their image than they do about
education. I have seen that more times than I can count, and faculty are
by no means guiltless in this. I have seen many really quite mediocre
(or worse) faculty cater to the worst of student behaviors in order to
ingratiate themselves, and often to put down their colleagues.
There are surely island of institutions who really care about standards,
but the overall picture in the past generation has not been a pretty
sight.