Until a few years ago MathOverflow front page had a short section at the top describing what the site is for. I am not sure when it disappeared or why but I would like to see it restored. Currently new users have no idea what kind of questions are appropriate here. One has to dig through Help center to find this info. This is not user friendly and most people would not know to look there. Adding a short "About MathOverflow" section to the front page should cut down on off-topic questions (calculus homework etc).
To add an alternative suggestion to Tim Campion's answer: Maybe it would be good to have a banner in the sidebar - similar as on Law Stack Exchange or Medical Sciences Stack Exchange. (And also on some other sites - see here: Which elements of the frontpage can be customized per site?)
The fact that some sites have such banners suggests that this is technically possible. (Probably the moderators would have to check with some of the community moderators or some other Stack Exchange staff whether this would be a feasible option also here.)
And, of course, there is a separate question: what would be a suitable wording for MathOverflow. But deciding the wording would probably be better as a separate discussion (a separate question here on meta) - let us consider this mainly as a "poll" whether going in this direction would be viewed as a good option by the MathOverflow community.
To clarify a bit better what I mean, I will include a screenshot from Law Stack Exchange:
And also from Medical Sciences: