With mine, the biggest issue was probably figuring out how the "throw the match" mechanic would work. Getting the combat working ended up going smoothly, but after the first day I was stumped -- I had an initial idea of having to please the mob by fulfilling certain criteria (like getting health bars to match up at certain percentages, or getting bonus points the lower the enemy's health got), but none of it felt intuitive or easy to explain quickly. In the end I kind of threw up my hands and went with a simpler score-based system because it was the easiest to code and the easiest for people to "get".