I'm developing a theory of game mechanics where there's a spectrum between "upgrades" and "options". Contrast, say, clicker games (pure upgrade) with something like deckbuilding in a TCG.
The question that keeps running through my mind is: is one players' experience of a game -interesting- to another player? Crusader Kings: yes. Dwarf Fortress: definitely. All these twee city builders coming out with good aesthetics and basically linear progression: definitely not.