I'll say that again: there's no such thing as too hard.
It's perfectly acceptable for a game to require players to have savant-like intellects, superluminal thumbs, and a latent ability to predict the immediate future.
That's the beauty of games. Doing what we never thought was possible. Threading the eye of the storm with perfect lucidity. That's the gaming high, and it's the reason we keep coming back for more.
The catch is this: difficulty must be a stairway. Games must start at the bottom and they must introduce each new concept incrementally. Every escalation in complexity and difficulty must be discrete, and each plateau of challenge must be eminently reachable from the one below it.
If you set the bar low and raise it by slow and unflinching degrees, you'll soon have players vaulting higher than they could ever have imagined.
This is part of the unheralded genius of Guitar Hero. From a beginner's standpoint the songs on Expert difficulty look impossible, but as you play through the game skill sneaks up on you and soon you're ready to go toe to toe with sequences that previously moved too fast for your brain to process. The songs themselves teach you the skills you need; if you haven't learned how to do hammer-ons by the time you hit "Jessica" on Hard, you'll discover an unthreatening environment that trains you in them just in time to put them to use in "Freebird" a couple of sets later. Similarly, your fingers learn simple note progressions in early songs that are repeated faster and longer as you enter the higher tiers.
It's also what made Guitar Hero 3 not quite as good as its predecessors. Note maps are apparently set out with the intention of thwarting players rather than challenging them, and ridiculously hard chokepoints are inserted amongst comparatively forgiving peers. I speak from personal experience when I say it's possible to complete every normal song on the Expert difficulty and still be blocked by the end boss of Hard.
There is no limit to what you can train players to do. Aiming high is the mark of a memorable game. But once you've set those goals, you then have to bring your players to them, and suceeding at that is the hard part.
The ambitious game builds a long and epic stairway. But in the end the good game - and I mean the really good game - walks beside you while you climb it.
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