On to the more critical part..
Classes
The three archetypes are enough since you can specialize them in different ways. But the skill system tends to make the characters either a little bit weak or one trick ponies. Sticking with only one type of weapon for warriors and rogues, or one line of spells for mages seems to be the optimal no-brainer way. It would be more interesting and fun if your axe specialist warrior would still be able to use a mace effectively instead of suddenly sucking horribly. Say there were enemies who are more resistant to slashing damage - it would make sense to carry the best mace as a backup weapon for those, if it would be more effective than your "trained" weapon. In short, specialization is always good but here it feels it goes too deep in an all or nothing kind of way.
Magic
The foundation and schools for the magic system are awesome. The casting combinations are fun, it feels different from swinging a sword.
However, Mages could have more spells to play around with. The one track specialization is at its worst here. Mages could have tactical choices or what spells (direct dmg / damage over time / buff / debuff) to use for best effect against certain enemies instead of just recasting one spell. Being a versatile 4-element spellcaster could mean you can cover more situations but with less potential. Now it just sucks, unfortunately.
More spell wishes:
Death magic - weakness, debuffs, lifedrain
Life magic - strength, buffing, regeneration (I like it how there are no instant healing spells, makes getting hurt more meaningful if you can't easily undo it)
Physical - force effects, pushing monsters around (or into a pit
Mind - confusion, mind control
Balance - anti magic, dispel, protections
Elements - solid and damage walls!
Tactical vs. action gameplay
This is just personal preference of course, but I think the game is leaning too much towards twitch-based. Some of the fights just require you to move around the enemy like a ninja and they never even get to attack. In that scenario all your carefully planned balanced character builds just fly out the window and defensive skills become meaningless. If you're willing to do the dance every time you can just put all skill points into attack power. In an RPG character builds should come first and twitchy action player skills second. Here it feels reversed, sometimes.
Multiplayer
Last but not least.. Who remembers Bloodwych on the Amiga? The first dungeon crawler ever with split screen multiplayer. How much fun was it to play with a friend?? We spent countless hours grinding through that dungeon. I hope Grimrock would have that one day.
Of course it would mean dungeons with puzzles designed for 2-4 players.. but single player could also have some kind of AI companions that you could order to stand on a pressure plate.. or a feature to split your team up and assume control of different characters... getting a bit far from the present here now but I hope there's all kinds of possibilities now that Grimrock is selling as well as it should.
Thanks for reading, can't wait to see what comes next!