I can’t think of many products that don’t have some sort of function indicator, except maybe the pet rock - as long as it didn’t have a flat side (inherent for setting it down on).
For Black & Decker, I designed a bubble level that sports two retractable polished nylon pads on the back and adjacent to those are two fixed, nonslip pads…so you slide the level at will and get it where you want it, but then you need to grab the pencil and you’re worried the level will slide just a bit, so you push against the wall and hear ‘snap’ on both sides as the two nylon pads retract in, now the nonslips are holding tight against the wall so it won’t move. Audible snap = won’t slip.
Also for Black & Decker, our field research found it was common for people to throw tools in their toolbox and run the battery down because they didn’t realize something was still on so I designed a little laser level that had backlit glowing vials in any of the on positions as a reminder to turn the tool off. Visual glow = uh oh, better turn that off.
Tactile indicators with feel or vibration are always cool too.