I must commend the puzzles on their logic. Between sudoku's and a support job where solving difficult issues, Grimrock 2 surely was one of the better brain teasers. All puzzles were fair, most were not terribly difficult once you learn how to think within the grimrockverse.
The secrets were far more difficult but most were telegraphed in a way that greatly reduces blind searching. Whether you could see an island or hear a door opening, missing secrets is usually the fault of the player. I had to bang my head against some of them, but overall I knew where secrets were before I solved them. By merely playing the game, I ended with 75 out of 77 secrets. That's awesome design.
I don't know the reasoning but personally, buried treasures seem to replace the irrelevant LoG1 treasures that only held value for a particular steam achievement. Finding buried treasure is one of the most awesome experiences when done right. Compared to zelda or fable, grimrock 2 clearly has the best treasure mechanic.
There are a few negatives too. These are all imho:
1) The legend of Kilhagan feels redundant. He doesn't compare to Toorum.
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The testament of the final boss should have been signed Kilhagan.
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neither do I get the trickster/island master link. I may have missed something but all three big bosses are essentially the island master. Makes no sense to me.
3) "armor is all over the place". My thoughts on this can be found in the relevant thread. The game could have been slightly more interesting by optimizing where armor can be found. Especially the mirror set should not be completed in mid-endgame area.