I'm a player, not a modder. I realise how difficult it is to make a good mod, and I appreciate the efforts to which others go to do this. I'd like to share my views on what makes a good mod, for the reference of those who are building custom scenarios. I'm sure that many players here will have similar views.
1. Immerse me: This is the premium feature of Amnesia, which cannot be over-emphasized. Appropriate lighting (most mods I've played so far are too well lit; don't give me so much oil, give me fewer tinderboxes, and force me to move into the corners of the rooms if I want to know what's there), good use of ambient sounds, carefully planned and well executed environments, all make me feel like I'm in the middle of a real experience
. Don't overwork the jump scares (slammed doors, blowing wind, shaky camera), it's much more realistic if you just focus on making me believe I am really there. My mind will then do most of the work for you; I will fear the shadow in the corner because I can't see what's there and I'm afraid of what might be.
2. Make me afraid, and keep me afraid: Amnesia worked so well because it introduced a sense of dread very early, and maintained the tension by making the player anticipate and expect
frightening events, even though the events themselves were typically far apart. Please use the atmosphere, environment, and narrative techniques, to make me feel constantly on edge. I strongly recommend reading this
3. Give me a reason to continue: I should have a clear reason for being where I am, and I should be motivated to continue by wanting to find out what is happening. I don't want to be motivated to explore a room just to find a key so I can get into another room and start looking for the next key. I want to be motivated to explore a room to discover more about the backstory, to escape a real or imagined threat, or to find something which can help me feel more in control (even if that control is only illusory). Use narrative breadcrumbs, but use notes sparingly, better if I enter a room with a communicative environment, like the animal dissection room in Amnesia or the dining room in the Cannibalism mod. A map could really give me a sense of purpose, and even better if the map is inaccurate or misleading.
4. Give me choices: The best part of playing Overture, Black Plague, and Amnesia, were those moments when I thought 'I wonder if that's the way the devs intended me to get around that obstacle/complete that task?'. Providing a branch of choices really heightens the sense of immersion. I like the fact that in 'Cannibalism' you can reach the second floor through unorthodox means, regardless of whether or not the editor had that in mind. This kind of experience is far more satisfying than the feeling that I'm moving on rails. Include as many interactive items as possible; I hate standing in front of a bookshelf without being able to move 90% of the books, and I hate the fact that I can't pick up a candlestick on a desk. Use many breakable windows, even if they don't lead anywhere (not only does it heighten the immersion, it makes me feel that I have more options, even if those options turn out to be dead ends). I would really love to see someone make good use of breakable windows, and indulge in a little rooftop exploration. Use outdoor scenes; if I can see a world through the window, it makes me feel like I'm really there (opaque windows remind me I'm in a tiny virtual sandbox). I was delighted to see this effect used in 'Cannibalism', it really enhanced the experience.
5. Give me puzzles: Puzzles are great for three reasons; they make me work for information (increasing gameplay time), they encourage me to continue (prolonging interest), and they enhance immersion (making the game seem more realistic). The good old 'pull-the-books-in-the-bookshelf-to-open-the-door' puzzle has probably been done enough already; try to think of something more subtle. Make traps as well, it's even better if I think I'm working on a puzzle which will help me out, when in reality I'm about to kill myself.
I hope someone finds this useful. I apologize that I'm not a modder so I can't put these suggestions into practice myself, and I don't want to sound like I'm lecturing others. The mods I've played so far have impressed me as much for the boldness of the modders in making the difficult effort to get their hands dirty with the level editor, and I admire that. I'd just like to contribute some ideas which might help them develop their work to the next level.