I honestly hate when games make you start the entire level over when you die. That's probably the reason I've never finished a level of Mario , the fact that each death makes you lose a ton of progress. I love platformer games but when I constantly keep dying and having to start over it really discourages me from continuing to play, so I'm pretty sure I'll go with a checkpoint system where the player only gets sent back to the most recent unlocked cage or found key, i.e. the last actual progress they made in the game.