A few years ago I started playing a lot of mario style platformers writing down notes including what I found "fun" (for the day I would get around to making my own platformer).
http://benjamin-meyer.blogspot.com/2010/04/what-makes-fun-ma...
Skip down to the "What is fun" section and you will find the first two items pretty much say when you die it is not fun. Skill level doesn't necessarily equal fun in a platformer game. That block that almost kills you and scared you? Turnes out it was scripted and there was no way you could have died no matter what your skill level was.
There are a handful of platformers on Linux and they all have the problem of being too hard. The games in the first few levels are so hard that the player quits before they can explore what the engine can even do or find the "fun" in the game.
Maybe difficulty is simply the easiest nob to twist when designing platforms which is why so many first time platform developer turn that first, but I would have to think that the fun found in a platformer is not from the difficulty, but in other aspects. No one would dare say that Mario 2 (japan version) is better than Mario3 because it was more difficult.