- An unambiguous way to describe things getting worse. Eg. hit points, conditions, etc. There needs to be clarity on "how much more do you have left in the tank". (As a GM. I'm ok playing without this, or having eg. HP hidden from me.)
- Support for my kind of fiction first. ie. rules don't engage until the fictional situation as descibed calls for them. "Describe what chracters do in fiction" -> "When situation calls for mechanics, use them" -> "Describe result in fiction". I'm ok with occasional rules first, but I don't like it as the norm. (Both as a GM and as a player. As a player this is central to my enjoyment. As a GM it's less important, but still get more enjoyment out of a games where this is true.)
- Bonus: character creation that is either a minigame or a creative exercise, not point optimization or a series of deeply interconnected decisions that you can mess up. Character creation that feels like work is not a deal breaker, but eh. (Both as GM and player.)
EDIT:
- As a GM, ability to create creature and NPC stats out of thin air without messing up the whole thing. Ie. necessary stat blocks must be small enough, and easy enough to understand that what I intended to be a cute rabbit doesn't lead to a TPK. Being able to go detailed is fine, but the minimum needed should be around 1-3 numbers, plus a single phrase or mechanical keyword. This isn't a 100% dealbreaker, but a strong preference. Good examples: HeroQuest, Over the Edge, Fate, Cypher System. Acceptable examples: OSR-style games.
- Bonus: support for wide "profession" skills. If I want to make a competent Naval Officer, I don't want to have to figure all the possible skills a Naval Officer realistically would have, especially if the game is 90% about going into haunted houses. I want to be able to write down "Naval Officer <skill rank>", and use that for all Naval Officer things, even if there's a more specific skill in the system. I can manage without this, but every game I've played that does this has been better for it. (Both as GM and player.)
- Originally I had Randomization, but then I realized that while it is my current preference, I would not mind playing or running Nobilis again, and I've always wanted to have a go at Amber Diceless...