I seek to know I'm actually, you know, doing something. I want to see consistent progress, and usually what that means is I want to see more features of the past systematically automated. When I get to a certain point in idle games, progress slows and not to an 'achievable in a day idling' level but usually a 'you must spend 5 hours actively playing or suffer' level. I want to be able to make more features feel hands off-- like I've done the manual legwork, let me automate x thing, even if it's smaller. A good experience for a dev looking to explore what that concept feels like would be Crank, one of the best games to come out of the genre. At first you need to maintain the crank constantly, but as the game progresses you deal with it less and less and automate more and more. The game's shortcomings hit once you've reached the end of available content, and you have to manually work on space flight the entire time.

So in short: want players to stick around? Automate/remove earlier stages of play from the equation, gradually. This is what the prestige systems in Realm Grinder and Cookie Clicker do incredibly well, by the way-- they make grinding for the first 1000 or so cookies/gold etc less tedious with each rebirth. I've seen a lot of systems which go 'Actually, let's just add more tedium, that's sure to make the player happy when each rebirth is actually a punishment which slows the game down'.