Played demo. I did enjoy it so these are more suggestions for an improved experience :D
First is on the main character sprite and animations that a lot may be a work inprogress but this is just of the content as it is. I see you are going with a very NES style game and I think what might improve the character is slightly changing the colors so parts of him do not blend in with the background. Mainly just the black parts of the sprite so details of the character aren't lost in the background(head and ankles) and he contrasts fairly well with the background like in the 2nd screenshot you have posted :)
Defintely can say add a swing sound to the sword to show the weight of it's attack and maybe some details to the sword itself so it isn't a giant hunk of grey. maybe some holes or a line up the sword to give
I think while jumping you don't need to have the weapon shrink as you jump. makes it look kinda awkward as it shrinks a lot. Wall jumping feels fine but could use maybe a few more frames of animation to make it a little smoother. The same could be said of the idle animation. It looks just a tiny bit too fast. A small nit pick
The music was really loud and kinda took away from the other game sound, but I did enjoy the music :D
Last but not least is the rock throwing dude. He could use a bit of a delay between his throws to create an easier opening and I think his rock hitbox might need to be shrunk by like a pixel or two. To make the opening you could do a small change with the rock throw had having it take a bit more time to throw the rock showing effort in the throw itself so it doesn't look like it's just flying out it's hands. This creates more of a fair opening and makes it looks 10x better. The monster hitbox itself may need to be shrunk as well. The one on the ledge before the boss fight was really annoying to kill. You can't hit the ledge and jump off it and attack without it "touching" you and hurting you even though you should likely be safe and then you can't attack.
Not sure about the Boss design but he could use maybe attack patterns instead of just flying and the air and hovering and falling. Shovel knight is a great inspiration for bosses like that. Like make him jump dive jump dive jump dive at maybe a higher speed for the jump and dive attack to trigger so it doesn't look awkward while your running around the room.
Can't wait to see more!