I'd say no harder than a strange dream on its hard setting, and that was stretching my patience.
...
Actually it's probably better to just say I don't really like twitch gameplay (you know, one errant twitch, and POOF!). puzzles are cool though.

ones I've played and really liked so far
Don't eat the mushroom
The Great Race
Harvest
Knytt on the Moon
Riku Island
The original level and expansions on the main website (though gustav's daughter was a bit hard for my tastes)
a walk at night
colours
most of googoogjoob's stuff
A walk at night

I've also played explore challenge, which was reasonably cool.
Environmental, Plot, and especially strange/weird/funny plots seem to be what I like right now.

Thanks!