an analysis of super metroid's level design and how it's used to intuitively teach the player what they need to know:
(level design as tutorial)http://www.gamasutra.com/blogs/HugoBille/20120114/90903/The_Invisible_Hand_of_Super_Metroid.phpThis article used to have a bunch of images accompanying it which made it even better but unfortunately they're gone now (perhaps archive.org saved a snapshot of the article with the images still there). If you've played Super Metroid a couple times you should be able to follow just the text, though.
breaking down the level design of sonic the hedgehog:
http://forums.sonicretro.org/?showtopic=28302identifies a "three route" system employed by sonic the hedgehog levels where three distinct paths through a level overlap. also suggests how good sonic levels additionally have special mechanics to keep them interesting, and more
(also, maybe more people would see this thread if it was just in the Design forum? lots of level design on a mechanical level can come down to being more design than art too)