Here have been my observations, based on responses to my projects, view counts, etc.
- Do not make your games difficult. If you want difficulty in your games, either leave it closer to the end of the game or have different modes (Like Easy vs Normal vs Hard). A game that is difficult at the start is a serious turnoff.
- Do not make your games too long. Your typical itch game should be complete-able in about an hour. If you want your game to be longer, it's a good idea to mention it (somewhere) that it will be a longer game. Even then, I would cap your full gameplay duration at 2-3 hours.
- Metroidvanias and other action-oriented games seem to do better.
- Heavy text-based games seem to be iffy; might work or might not.