Ok so here is what I found:
Bug: Holding a side direction in the main menu with a sidemost option selected will make Scrapgirl freak out. Eventually, all this turning around and running from side to side makes her sketch graphics to turn completely black (like a shadow) while staying at a weird, half turned angle.
Suggested fix: disable main menu "wrap around" to make the spamming that makes this posible not available to the player anymore.
Bug: Spinning cards in their overwold form sometimes display a completely black backside, probably due to lighting.
Suggested fix: Give these cards their own light source to both overcome this and give them some "importance"
Bug: Scrapgirl might vibrate or rock slightly back and forth in some complex level geometry.
Suggested fix: Assuming you are calculating Scrapgirl's angle in realtime with some kind of normal calculation: don't. You are better off establishing simple collision maps for otherwise visually intricate objects.
Bug: The second tree (the ragdolly one) in the second room freaks out way too easily.
Suggested fix: Tone down realtime physics and give it an animation instead.
Tidbit: Mr "you fixed the bridge" mouse guy just casually floats across the gaps (snort)
Tidbit: Never though I said this, but this game needs more invisible walls,specially on the front and back edge of the stage. I find myself constantly falling even when paying attention. (The sound that plays when this happens is amazing, though)
Tidbit: Speaking off, the "you fell off" mechanics need some revision. For one, camera needs to stop following the player. Otherwise you start seeing floating level geometry and the innards of the level design. I also strongly suggest getting rid of the "blackout fade" effect and instead implement something more in the lines of Paper Mario TTYD (Where you get your but munched on by some chomp fish and fly back to the next checkpoint).
Tidbit: Speaking-speaking off, the checkpoint system is at a whack. If I'm coming from the right and fall in a hole I should spawn on the other side, not be "rewarded" and teleported to my destination.
Bug: In the paper plane room, the sign that tell you not to speak to the cat displays it's "?" horizontally flipped.
Bug: You are able to pick the plane powerup more than once, as it keeps respawning upon re-entering the room.
Tidbit: You walk SO SLOW in the 2D background section.
Tidbit: Need a "you got something while holding it up" charade for items that are cards and "important stuff" like the fruits.
Tidbit: In the rainy tornado room you could benefir from forcing the player to pick up the card. Don't be afraid to gently trap the player to force them into learning something! If you can't, remember to implement a camera pan towards the missed crucial item upon entering the room right before allowing the player to move (See Luigi's Mansion)
Tidbit: The second gap in the second room is a leap of faith. Mark down a trayectory with coins. You could also benefit from using Mr Mouse again, making him complain about a second missing bridge the player needs to reactivate and being all "Now what?". Also the guy crossing the bridge should really be a mini-cutscene where the player looses control for a couple of seconds.
Tidbit: Need to "gently force" the player to practice the tornado+jump technique in a safe place, probably design the "trap" in the rainy room so the player can get in but needs to do the special jump to get out.
Tidbit: When moving fast the camera fails to catch up. For example, if the player moves right very fast they are on the right side of the screen. What should really happen instead is that the camera looks ahead more, placing the player on the left side of the screen and allowing them to react to incoming level.
Tidbit: Careful with finicky platforming in these game style: it can get tedious FAST.
Tidbit: In the rainy room, the shallow water on the right looks inviting, like it's not deep enough to kill you and that you can probably just walk inside without fear.