(Seriously, what is the point of spoiler tags in this topic? It should be read by those only who have finished the game.)
1) What puzzles did you like the best? Why did you like them?
I think that puzzles are a strong point of Penumbra: Black Plague and you don't really need to evaluate your riddle making skills, because you're already mastered that.
Most of the puzzles were unique and interesting in nature. It's marvelous when the solution is in front of you but yet you can't see it because you haven't put the pieces together in your head. (e.g.: the trusted blood puzzle, the fire extinguisher system puzzle, the synchronize the items with the ones on the painting puzzle, the thermostat puzzle)
2) What puzzles were the hardest to solve? Why were they so hard and what kind of solutions did you try before finding the correct one?
The pipe puzzle was quite hard, because I didn't assume that I can break the pipe by simply throwing a box at it.
Before that, I tried to squeez the curved pipe piece into the system.
I followed the pipes in both directions looking for clues or valves.
And tried to hack the nearby dashboard then I started throwing a box at it, which will eventually lead to the final solution.
The scaffolding puzzle was tough because of the misleading seesaw on the ground level. Later I read that it had it's own purpose (getting up to the switch) but I did that in another way.
(I lifted up the depressed end of the wooden board, that is to the right of the switch making it laying horizontally on two iron pipes, then I hoarded two boxes on it, and from there I was able to jump on the platform where I can pull the switch.)
This was the only puzzle where I had to use help.
The chasm filling puzzle was simply annoying because of the cold that forced you to run back to the shack periodically. I remember when I was dragging the last element towards the chasm that should make it possible to jump through, I realized that if I screw it up somehow I'll have to start all over again... I got really scared
And if the boxes fell in a wrong way you are out of luck too.
Before solving that puzzle in theory I was trying to connect the two ends of the chasm with those long thin pieces of wood.
The kennel puzzle wasn't entirely obvious either. First I gave up. Later I came back and tried everything, even hid in the dog shelters so that maybe they got so angry they come out and kill themselves with the light. I was poking the body of the dead dogs. I tried to hack the electric equipment. Even when eventually I found out that I'm supposed to block the holes in the walls, first I used the barrels instead of the metal crates and got gnawed upon.
3) How did you find the pacing in the game? Where there any particularly slow moments or some moments where too many things happened?
It was okay.
Maybe except the part when you get through the emergency door in the chemical lab. The camera corridors were very pointless and out of context.
I thought that it just got made to show off a feature of the game.
4) How did you like the variation of events and gameplay elements? Could there have been more diversity or should there be more focus on the gameplay?
The events were well placed and good, and thankfully there were not too many of them because every scripted little scene is a chance to break the immersion if it isn't done right.
I liked how Clarance made the door disappear and then how he made me think that a corridor became a dead end for example.
5) How did you like the characters in the game? Was their voices fitting? Was the voice acting good? What character did you like best / worst?
The worst is, the female scientist, because her whole character was stupid and forced (not her voice, I liked her british accent).
And Clarance is the best, though sometimes he talked even when he shouldn't have to.
6) How did you like the ending?
There was no big boss monster, thanks for that. The Hive mind trials were a bad idea in my opinion, probably a recycling of something that was supposed to be something else.
Too much talking at the end (and it wasn't worth listening to anyway, just the usual obscure yaking of the all knowing nemesis) I found myself running around in the top of tower waiting for the hive mind to shut up.
I tricked the second trial by the way by jumping on the top of the wall, thus elevating the number of glitches used in the game to three. It's the most I found in a game without even looking for them. The first was (in the demo) on the trusted blood gate corridor when I clipped through the fuse box. The second was not using the seesaw to get up to the switch.
But hurray for the unusual no happy ending and for the room that was slowly eaten by darkness.