This is a WOW game.
This game doesn't have cutting edge graphics, the audio is slightly better than average, and the genre is as old as, what, the 1980s?
This game isn't great because of some stupid parlor trick. It's good because it's a near-perfect implementation of a specific class of game; the old PC dungeon crawler.
If you love finding secret wall switches, solving puzzles and finding loot, and eating a giant roasted maggot-on-a-stick because you're desperate to eat, then this is the game for you.
If you want a fantasy FPS with lots of jumping and airducking, this isn't for you, GTFO.
Legend of Grimrock doesn't try and improve much on the old classic dungeon crawler game design, I think that works well for it, and I don't think they do anything really new or crazy that hasn't been done in dungeon crawlers before. They just put modern graphics and sound together into a game and implemented it well.
The game doesn't have any music beyond the title screen, but the ambient background through the game is pleasant and detailed. Turn the volume up, because sounds of the dungeon will alert you to nearby foul beasts and secrets.
The background story is compelling and interesting, but nothing too deep. Secrets are found in the dungeon, and a reason for being is evident, but they leave the details up to your imagination, which is probably for the best. You can't climb back up. Your only way out is down. The story is your journey along the way.
The last few levels are spooky and the game has a very fulfilling final battle.
Secrets are plentiful, so pay attention. There were a couple of puzzles which had me really baffled, including two that were required to proceed with the story. I don't recomend ruining the challenge for yourself by reading a walkthrough online, but if you get stuck in the story, it's there for you. The best thing to do is sit and think. With two specific exceptions that I can mention, most of the puzzles are evident if you just use your brain for more than thirty seconds.
I do have a complaint regarding the final puzzle in the game (Minor spoiler warning): There are four items which need to be placed around a single block in a room. Two of them are pretty easy to figure out, but the small gear in particular has a non-obvious arbitrary placement location which should be fixed. Even if you have the right item on the right wall, you have to click it on a very specific small area that doesn't offer any clue that it's the right part to go in the right place.
Also, the level 9 riddle room had me really stuck for awhile. I just didn't get the clues and had to randomly guess at it until I figured it out.