Comparatively speaking, horizontal is easier to handle than vertical in my opinion.
Consider pacing, for instance. If you have a run-and-jump game where you have to move primarily horizontal or primarily vertical, you typically can accomplish a quicker pace going left and right as opposed to the other. Why? I suppose running is more consistent than jumping; when you jump, you have to contend with gravity, for one.
Trying to establish any kind of flow to vertical gameplay seems tricky. Yeah, we can have wall jumps or bouncing pads, but think about it. You still are fighting the constant momentum change instigated by gravity while in the air for extended periods of time. It’s not like running, where you can maintain momentum when you have a clear path; you have to keep finding ways to fight gravity’s pull and move up, move up, and move up. You don’t typically flip the switch and bang, “I’ll just fall up instead of fall down“.
It, of course, raises some interesting scenarios. What about multi-jumps? Well, you would need A LOT of multi-jumps or a few multi-jumps with A LOT of power if you wanted to maintain some form of level momentum. Games have used bounce pads, but often those cause sharp changes in vertical momentum as opposed to keeping it consistent. Jetpacks? A possibility, though figuring out the right amount of thrust would be crucial to that working. What about a gravity-flopping mechanic implemented in a traditional run-and-jump platformer? Hmm…
What do you think?