The only real complaint I did have was the level design. At first, I really did thnk the levels were randomly generated. Randomly twisting corridors, dead-end rooms that made no sense and arbitrarily placed puzzles all made me think it was being generated on the fly. Unfortunately, it wasn't. This has two effects, the first being that replayability is limited - Legend of Grimrock is the same every time you play it. The second effect is that, in my opinion, they're designed like someone pulled something out of their ass, smeared it on canvas and said "that'll do pig, that'll do". Still, provided you never ask "just why the hell did someone build a room with this kind of trap / puzzle in it anyway?", you'll do fine.
As I've said before, the puzzles themselves are very well designed - with plenty of adequately obscure (but not too obscure) clues found nearby (if you look hard enough). It was only on Level 9 when everything went to shit for me where after successfully getting through the game without help up until that point, I had to google for assistance. Working out what to sacrifice, missing a switch in the wall and just how exactly one should slither… In hind-sight, the switch should've been obvious but after the slithering puzzle (a note I had written off as a clue on how to fight the Big Bad that the game continually hinted was coming up), I honestly thought I'd missed something somewhere. And I would have been completely stumped at the sacrificial altars without reading the help in the forums.