ladders:
i'm planning on adding ladders! i have the same feeling about vertical movement. i plan on adding at least one character who can't jump at all (something in the vein of lode runner) and i think ladders would be useful both for that character and for the other characters.
i think the way i'll implement it is by adding a ladder to the entity palette. any tiles you paint ladders in will be climbable. ladders will have a default appearance, but you'll be able to swap in any sprite you desire, so you could make climbable vines, bones, etc. if you desire.
i'm also currently implementing tiles which push you, which can be used in various ways to move upward.