tinyE: You are going to have to explain that one to me.
Telika: It's what distinguishes side-scrolling platformers I can play from side-scrolling platformers I can't. For me, movement is left right down jump. I can't wire my brain to hit a different key (space, J, or whatever horror they want me to) when I want my character to, well, hurl itself upwards.
It's exactly the opposite for me. I want movement on the arrows and jump (ideally) on the spacebar for the left thumb or (bad but tolerable) on the right control for the right thumb. When I try to use the fingers on one hand for moving AND jumping, I die.
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The worst thing ever is wasting my time, typically by requiring a pointless waiting period before I can retry something that's meant to be retried. E.g.
La-Mulana: no checkpoint before bosses (it could've been an ephemeral checkpoint: lose a boss battle and you can retry, decline to retry and go load a legit save elsewhere instead).
Titan Souls: same, but the game is all bosses. My precious
Teslagrad has checkpoints everywhere and lightning-fast restarts after death... but there's a huge room with several trick jumps that's impossible to die in.
Another thing: no good endings. Be selling me a game, you implicitly claim that the game is worth playing, as in, you made it so that people would pay their time/money for the privilege to spend even more of their time/money on effecting changes in an imaginary world. If those changes are invariably for the worse, I shouldn't have played the game in the first place, which means I was scammed out of time/money.