I'll take a shot

• The shading is really kind of wonky? Mario has this weird reflected blue light on his overalls that doesn't make sense, and the environments are super bright and sunny, but the sky and the distance fog (dunno the technical term) are weirdly dark.
• The current model for Mario has slightly different proportions than the 64 model, and the animations haven't been changed, so he moves erratically. I'm pretty sure he's also smaller, and that leads into:
• The stage is goddamn huge and not in a good way. In the original, yeah, you had to go a ways to get through the course, but it was fun and jaunty. When I tried the browser version a while back, it was waaay too open. The pyramid thing the cannon comes out of might have been that tall in the original, but with this Mario it looks way too big. The four posts where the Red Coin star winds up feel too far apart. There's too many empty spaces.
• Looking at Mario 64 again with this playthrough has made me realize how strange a lot of the texture choices were back then, but everything in this demo is FLAT. The mountain has a teeny, tiny brown mottled texture that, when spread out over the entire wall essentially becomes solid brown. Same with the grass. The grey rocky slopes and the wood planks are okay? But there's nothing to really look at. Compare this shot of 3D World with the video. The grass has a chunky, leafy texture that overhangs the cliffs. The walls have mineral streaks in them and lines of rocks. These guys borrowed current Mario models for Mario, the Goombas, the springboard, etc; they should've grabbed some textures while they were at it.