So after downloading and playing this second demo, here are my thoughts!
I made it up all the way up to the boss fight (on hard skill), yet have not gotten around to beating it yet. I enjoyed the monster, like a version of Chthon even more Lovecraftish. The main problem that was quite frustrating for me were the boilers. I stood at the corner where the RL was, yet when a boiler exploded, it still managed to instagib me. Earlier I tried confronting the monstrosity once again, yet the explosion reduced me to 50 health from 94 armor and 200 health... I think that is a bit harsh considering how far away I was from the boiler. I could not tell if I was really hitting the boilers as well... I ended up shooting towards the top where I noticed some different yellow particles coming out, and that seemed to do some damage. I am guessing that to properly beat this fight requires some good use of the grappling hook.
The grappling hook behavior I liked, the grapple points made it such that the level cannot be "cheated", like the usual Quake grappling hook behavior (hook off of anything). Agreed with Bloodshot there.
The sound effects... still need some work in my opinion (granted it is a demo, maybe they are just placeholders). Particularly some oddities with the lava sizzling noise from QuakeII. Nothing huge, but as I was grappling up to the grenade launcher they seemed to cut off almost instantly, rather than becoming faint in the background. The slight "merging" of QuakeII aspects (textures, lava, machinery), I didn't particularly like... Its not very Quakey

. Sometimes I almost felt as if I was playing QII (the armor shards, health vials) I enjoy Quake II, when I'm playing QII.
That aside, I did like the behavior of the monsters and some of the new additions. I reloaded the level one time and found myself face to face with an Elder Ogre... Reminiscent of ID's earlier ogre designs... Nonetheless it gave me a beatdown, which is why they seem so rare

. The new behavior of the scrags I enjoyed, they almost seemed like ghosts with their teleportation. Makes them much more of a challenge (and more scary). Also, the resting shambler was a neat touch

.
The sliding box puzzles... I thought were OK.. it took quite awhile for me to complete the first one, I think they should be able to be moved a bit faster. Jamming up the doors with the boxes was some good design. My thoughts are that the puzzles should be kept to a minimum, and they should be able to be completed fairly quickly. If not quickly... then have bits of action interspersed as puzzle actions are completed.
Now the level design... the brushwork... I loved. Tons of attention to detail all over the place. I enjoyed how I would suddenly be in an unfamiliar place, then be transported to a room which I recognized from e1m6. The level design far surpasses games such as Half-Life 2, Bioshock, most overhyped "mainstream" games. I'm eagerly awaiting Gloom Keep
I had a few problems with the engine, as others mentioned, the lava flashblend still staying after I died and reloaded in lava... Also savegames didn't seem to overwrite. I had filled up the save game area and once I tried to save again they didn't appear. Had to manually load with "load s13" in the console. Don't know if this is engine or QC problem, but as the instructions went by, if I clicked the mouse once, they ran through very quickly and I was unable to read. I'm guessing the behavior for this should be to skip through each message. I did like the colored lighting
I feel as if I'm forgetting some things I wanted to mention, but overall... I'm glad to see Remake Quake still going. Some really cool design elements and great ideas. I had a good time revisiting a new e1m6. A good christmas gift this was. Thanks to the RMQ team! Hope you can make some use of my thoughts here...!