Thanks a lot for the feedback, it'll be helpful.
The next demo will definitely have more forgiving scrolling and fall damage, and I'll look at increasing the jump speed. The keyboard controls were just sort of thrown together to be easily reached with one hand, I was planning on mapping some of them to the directions, so I guess I'll do that for the next demo.
The fixed jumping is very much influenced by classic Castlevania. I like how that style makes the game predictable and gets you to plan your actions and know how the game controls, and it makes the game different from a lot of others, so it won't be changed much. You can actually slow down your horizontal momentum a bit by attacking while in the air. I might introduce more air movement at a later date.
After you got to the end your jump height was increased, that's why you started taking fall damage from normal jumps-- in the next release that will be more evident, and it'll be clear how fall damage fits the game further down the road. I'll also try to make a more proper ending for the next demo, or the one after that.
The stairs being able to be walked through are influenced by Castlevania too, but they're also important to player choice, which is something I really want to explore with this game. I'll have optional platforms that are only reachable by stairs, and I'll probably have branching paths in some places (levels themselves will definitely be branching).
The player's hitbox for enemy collisions actually is smaller than the player sprite, but only by a few a pixels; I may shrink it further in the future.
edit: will also look at size presentation and full-screen support