The only thing I don't like about the game is also one of the things I enjoy about the game: the combat.
Initially, I was very satisfied with the combat system. It's like a simplified RE aiming system. You hold R to target, then use the control stick to choose head, torso, or arms, then you press the attack button. Quickly releasing the R button and pressing it again switches targets. It's simple, it's fun, and it works.
However, short of some boss battles (a few of which took me an hour or so to get the patterns down, no thanks to power outages we were having at the time), there is not much variety. To get the final ending, you have to beat the game multiple times. By the final playthrough you'll pretty much see the combat as a chore to be completed in between puzzles and cutscenes. The puzzles aren't mind-boggling, they are mostly "gather the items" or "use your new power" puzzles. The cutscenes are different enough for each playthrough in most places to be worth it, and the final ending scene is pretty cool if you understand what it means.
So really the only thing that detracts from the game for me is the monotony that comes from killing zombies over and over. My R button is getting worn and it caused some problems with targetting from time to time, but that's not really the game's fault.
I guess another gripe is that once you get experienced at combat you're not going to experience the sanity effects much at all. If you want to experience the most possible sanity effects, decode the following spoiler by reading every third letter start over at each space):
cdfhdfogyouistee ewtsxhswe dfXjkeiulng'wsldxoswtszahjtuuh (fsgczrreeszeren) aeavsrj7thliiuf,lakicojt dxfreidwrcjsuht
Everything else was spot-on and I really don't think any Gamecube library should be without Eternal Darkness.
If you feel like reading hints, then read the following sentence: except in one scenario towards the end of the game and against one specific type of enemy you don't want to get near, always favor melee weapons to guns and ranged weapons.