This game is amazing! The lore, the environments, the soundtrack, the dialog. Nice Undertale reference too :-)
The concept of the locks being knowledge from dialog is pretty interesting and well executed, though sadly I found it a little confusing. It took me roughly 10 minutes to pass the tutorial area, mainly because I didn't understand that I just had to talk to as many people as possible, but once I got that, the confusion was gone. Maybe a similar arrow to the one that appears above doors above NPCs would help?
On the topic of that arrow, it only seems to visible when you're in the very middle of the door / interactable. I feel like making the arrow visible more of the time, especially for those doors that initially just look like a black patch in the background, would be a great help to the obvious visibility issue so many people have already mentioned.
This is very specific, but when you go to the ending well beforehand, the invisible trigger that makes the player turn around is very confusing, especially when it happens in air. Even some "I don't want to risk falling down that well" self-dialog could help.
Also, as my experience with went, HP served literally no purpose. The rooftop pits are instakill, and the only other thing that I can think that might damage you is the person outside the casino you attacks you for winning, but I just jumped over to the screen transition after selecting "Try to run", so I can't be sure.
I did happen to find two bugs that weren't mentioned by anybody else. Firstly, and most gamebreakingly, you can still move your character when in elevators, making it easy to trigger screen transitions, which effectively make the camera move to the next elevator from the screen you just triggered, allowing you to see out of bounds in the hotel and other areas. (Try trigger the hotel elevator and instantly walk left - you'll see what I mean.) While in this state, you can re-trigger the elevator, making the elevator transition infinitely. You can also trigger those scene transitions that turn your screen white, leaving your character back in the normal state.
On the topic of the hotel, the second bug is that you can essentially break an entry and not pay in the hotel. The trigger in front of the door only slows your positive X velocity, it doesn't stop it - so if you take a run up from the other side of the room it's very easy to break straight into the room and access the savepoint without paying.
Also, that stair bug mentioned earlier did happen to me once - I'm pretty sure the bug is triggered by jumping and pressing up to trigger the stairs in mid-air. Or at least that's how it happened to me. (Oh, and by the way, this is pretty obvious, but when you go down stairs, your character points up the stairs. Looks kinda wrong.)
Sorry for that extensive nit-pick-list, as this really is a great game that I thoroughly enjoyed. Thanks for making it!