Don't forget the (in)sanity effects. Those were pretty novel, unless you were playing the game properly, in which case you never saw a single one.
IIRC, the game grants enemies the ability to cause 200% damage to your sanity meter if you go too long or something like that. Although once you acquire enough spells, you basically can't be killed or made insane if you can remember them.
I ended up beating the games 3 times to see the proper ending. Such a drag, gameplay is so boring.
This is the worst part nobody ever talks about. Even the one time through end game is horrible because you have to work through all those portals over and over AND FIGHT DURING THEM BECAUSE SHIELDS UP which grinds the game to a complete halt right as you've uncovered 90% of the mystery and are awaiting the last piece.
Though I guess that's probably because most people never tried to finish it, let alone played it.
That said, the "complete" ending is totally worth it. And by worth it, I mean should be watched on YouTube or read on Wikipedia because it's the cleverest twist of the game.