I guess the most important feature from my perspective is a physical keyboard. My most common "support request" is that the person wants to use the phone dialing app and can't figure out how to return to it or launch it. So I want a dumb phone or UI, where the dial pad is always there and always works. The only other feature I want is a clock, that should also always be on screen, that shows the time and date.

Everything else, such as camera, messaging, calendar, contacts (except maybe a few predefined contact buttons that are always there), etc., should all be absent or inaccessible. And, I have gotten the person a digital wristwatch for xmas, so that might relieve the clock requirement.