When you hold down and move, your character walks or runs normally. When you run into a set of stairs connected to your current floor, you automatically walk up them. I vote that if you are holding down and moving, you should not go up any stairs you bump into, and instead walk right through them.
Even just holding the down key would be nice for this in my opinion. Needing to hold the jump key too is a bit of a bother. The down key would basically check if the player isn't colliding with a platform, but while holding the key then collides with a platform. If that check passes, (ie. collision state goes from false to true) then the player passes through. This would be, at least, mostly consistent with how platforms already work. Right now I think it is associated with falling/gravity, but could easily be changed to look for a state change instead. This gave me an idea for another thread about platforms actually, there is yet another option.
I think holding direction + UP is the best, yes walking up stairs automatically when moving -i hate it.
Needs a poll - 1. Keep it as is 2. Go up unless expressly holding down(default key: s) 3. Don't go up unless holding up(default key: w) I'd vote "Go up unless expressly holding down(default key: s)" because that's mostly what I'd want/expect. I'd find requiring me to hold up each time to be annoying. And it is one of those things that would be harder for those newer/haven't gotten the controls down yet. They'd be wondering "why the stairs don't work." But the OP is right that we should have an easy way to not automatically follow stairs upward. Making you drop platform after the fact is fairly annoying and should be unnecessary. Of course it also looks odd.
You can't, no, however you can crouch while standing still. I imagine this setup would become really quirky feeling.
Technically you don't even need a way to say "go up those stairs" because we can jump into them if we want to go up so if the default was "always continue forward" it'd be still usable.
It's nice to have the integration of auto going up stairs. Castlevania eventually got that to work very nicely by holding UP when you want to go up. The tricky part is when you are on an upper level and want to go DOWN stairs as opposed to walking across a platform.