Getting all of the stuff in Metroid Prime 2. The game came out in '03 and I still haven't even completed the story yet. Not that's it hard, it's just long.
it all depends on the skill of the game you play. some games are easy to explain for some people, some are not to easy to explain. whatever game you're good at in a nutshell.
Simon's quest from the cstlevania series. That game has so many items to find, the townspeople lie, and obscure secrets. There's so much you can miss, a good walkthrough would be helpful.