Hahah, it's not like I don't occasionally post here (okay, maybe not as much as I used to, but whatever). Also shoutouts to Lucid for making the tool that's going to let me even work in unity in 2d to begin with.
Anyways, thanks a lot! As popular as I Wanna Be the Guy stuff is I'm semi-self conscious about it. Like, I always worry it's going to come off as arbitrary crap to people that anyone can do. Even though I know from experience that I have a good grasp on level design and psychology, I'm always worrying that "the next time I'm going to fail and fall flat on my face".
Anyways, I can give some advice about level design. Not all of this is universally applicable, but it helps.
Make your stage look like something
You don't have to be realistic or worry about little details, but if you look at IWBTG, even though even that is pretty sloppy, it has PLACES. The levels are solid structures and generally not (though not always) just free floating challenge rooms that can be anywhere and those that violate that sensibility exist to be jarring. Now again, IWBTG has a lot of weirdness in it's map layout, but if you compare it to any fangame, it looks a million times more sensible. Look at a Mario game like SMB3 or World and then look at a hack for either. What's the difference? Most SMB stages feel like places. Mountains, hills, coasts, deserts, caves.... and the hacks all have very arbitrary structures like pswitch mazes that don't represent anything and make it feel like... well, a cheap hack.
There is also more to a stage than the individual changes. How and why you move (even if these movements are ultimately unimportant) is very important. Use things like easy jumps and easy enemies to keep pacing. Pacing is suuuper important. The "shape" the player moves in a particularly level helps them remember the area makes it feel different from other areas. If you're moving only straight, that';s boring, but if you're going all over the place you're engaging all sorts of parts of the brain used for spatial awareness and that's good! It's stimulating to have good, active level design where the player is constantly doing something, even if them doing it is merely a formalit (IE: Jumping over a pipe uncontested).
Also aesthetics matter. You differentiate areas and help infer what sort of logic an area is going to follow. If a skyscraper level has the geography of a cave, it's going to feel wrong (unless it's some sort of ruined building or something).
Now this can go as far as you want. If you're making Bomberman, theming and level design are really minor. If you're making a metroidvania, you wanna pay a LOT of attention to even the dumbest details so you can get the maximum immersion possible. Anyways I'd break this down into...
Try and hit those four notes. There are other notes to hit too, but thos are the ones I feel I can talk the best about.
As for psychology, I can't really share much. I work from the gut on that, but I will say the secret there is to act and predict players with CONFIDENCE. Even if you your self lack confidence, just go with your gut and pray. Second guessing your self can be a curse.