Love this! Glad to see someone keeping 63 alive!
I played through the demo twice, and here are my thoughts:
Looks: it's beautiful. Koopas need more expression but things mostly look amazing.
Feels: restrictive, jumps feel weighed down and FLUDD as well feels like it barely lifts you.
Glitches, camera freaks out when I get near to the top or bottom of a level's edge. The transition to the area with FLUDD begins with Mario far off camera to the left, and for a while I thought I had disappeared.
Other: I liked that you gave the signs names, very fun! I think you should let the camera follow Mario a bit higher and lower, which may take care of the glitch, but regardless he got way to close to the bottom of the screen when you take the underwater passageway, and if you climb up the hill over the waterway Mario can jump to the top of the screen. Consider giving the camera more space to keep Mario fairly centered.