Wow. Everyone knows, of course, but it's taken me this long.
Incredible atmosphere. Fantastic controls and gameplay. Masterfully put together. Rewards almost everything you do in the game, whether it be skill-wise, exploration-wise, or just general play. I will admit getting lost a couple of times though - even though it is very good with its hinting at where you ought to be exploring - it's just that some areas are very large, and some intentionally confusing or 'loopy'. I don't see this as a detraction at all though.
I'm shocked by how fair it is, too. I was really expecting it to be a punishing, evil husk of a game, but in fact it's super friendly if you are willing to take the time. But it'll give you what you deserve if you over-extend or push too hard.
Absolutely stood the test of time, and I can't see it ever losing that.
I look forward to further play-throughs (of which I'm sure there will be many).