(1) Luigi seems to be missing a special jump animation for when he's holding something. Just uses the normal jump animation atm.

(3) I honestly just find myself using Luigi over Mario at all times. I'm not really sure how to fix this, but it may be something to consider.
Mario has basically no advantages over him aside from traction. It worked in NSLU because you..couldn't exactly pick Mario and the levels were harder to compensate for the ease, and in SM3DW because you had a whole host of 5 characters to pick from with their own abilities.

(5) In a room in a red pipe in 2-1, some Swoopers are positioned as if they were 'inside' some slopes.
Not a big deal, but it looks a bit strange. In fact, a lot of the bigger Swoopers in the level seem to be positioned 'inside' the ceiling?

(6) I feel like there's a lot of...unnecessary areas in Pipebreak Cave?
Like the part where you have to go into a pipe cannon, into a very specific area where you just hit a ? Switch and get back into the previous area, which unlocks the required red switch to progress.
It's not challenging or anything to get to the ? Switch, it's just...there. There aren't even any score bonuses on the way up. A bit strange imo.

(8) The wagons/minecarts seem to suddenly lose their hitbox during the 'bouncing animation' upon landing... allowing me to pass right through the damaging carts unscathed.

(9) A minor oversight, but in the wagon level in the section right after the two loopdeloops where you bounce off walls, you can bounce back onto the loopdeloops, and do the level backwards. Don't know to which extent since I couldn't get very far, though.

(10) I haven't played NSMBDS in a damn long while so maybe this is in the original too, but... You can just hit Bowser Jr. while he's down. Do that 2 more times and he dies in a few seconds. A bit...unfair? Shouldn't he be given some invincibility frames once hit?

(11) The tightrope near the start acts a bit weird when you stand at the edge of it.

(12) I don't think that Lakitus should be invulnerable to hammers. I mean, come on, they can kill Big Piranha Plants, the Cheep Chomp, etc... why not Lakitus?

(13) It's a bit unnecessary that 3-A gives you the Fire Flower...and then immediately after, gives you the Hammer Suit.

(14) Squigglers act really strange if you stand behind where they spawn. I don't think they're supposed to be placed on a horizontal pipe which the player can actually stand on. Maybe block the top off, like the pipe on the right?

(15) It's really easy to clip into upside down slopes underwater. They're very buggy.

(16) The ? Block near the end of Icecrack Citadel always gives the player a Mushroom. Probably intentional,
but just in case, I wanted to point it out.

(17) The bombs in the above Bowser Jr. fight were more...helpful than harmful for me, haha.

(18) ...lol

(19) You can actually temporarily move the entire level's collision slightly to the right using this extremely buggy 3-platform spinner. Also, this is how far I managed to get it.
You just have to nudge yourself in the little hole between one of the platforms and the wall in just the right way.

(20) The usage of the snow-from-branches in 5-Castle looks...really strange, with how the snow is nudged into the ceiling and how it jitters for no reason before falling. It's meant to be used on trees, not castle ceilings...

(21) King Boo can be completely cheesed with the Hammer Suit. Also, this.

(22) With the Hammer Suit on climbable ledges, you can throw a hammer while on the ledge. This wouldn't have been a problem, if not for this: if you're hanging on the ledge and throw a hammer, it slightly bugs out and instantly forces the player to be 'on' the ledge instead of hanging from it.

Not intended, so this means you can clip through when there's a solid block right above the ledge by just hanging from it and throwing a hammer.

(23) The Blue Shell/Hammer Suit shell ability acts really strangely with the Mini Mario space physics. Kinda fun so I'd suggest keeping it in, but I wanted to mention it anyway.