...If I were high on a mix of LSD, crack cocaine, krokodil and vodka

1. Too much boring, bland and flat space.

2. Unvaried terrain. It's all the same. Read point 1. - Bland, flat terrain with nothing fun or special about it. No flair.

3. Barely any terrain at all? I mean, the terrain consists of a few ground tiles. It's like the original
SMB, but with barely any fun obstacles or anything of the like to back the bad terrain up.

4. Way too many tiling errors to count. In most of the levels, there's LITERALLY NO WALL OR FILLER TILES. Sprites are cut off, mushroom stalks make the mushrooms look mutated because they're popping up around everywhere. You can't tile towers properly, either. (but barely anybody can for some reason, so shrug)

5. The level design seems to just be bland and thrown together without any thinking process behind it. It's boring and flat, and the player will feel bored just walking through a completely dry and flat land. Boring weird mini-mario pipe joint... Shapes, aren't any obstacles, either.

6. The levels have no point to them. I mean, you just walk in one direction. There's no thinking behind where to go or what to do. No theme, gimmick, or flair to your levels.

7. Mario Maker-esque gimmicks. P-Switch bridge? It can be nice if used in a minimal fashion and executed well... But it's not.

8. No bug-testing. I mean, lone small fireball in the middle of nowhere? And you don't even give a SINGLE fuck about all of those like, ehm, BILLIONS of tiling errors scattered all about?

9. Precision jumping. SMB1 had this. Nobody liked it. It's a bad level design practice. There you go.

10. Completely random warp pipes.

11. Are you insane? I think you have to be insane to NOT see that pretty much almost ALL of the wall tiles (and some of the corner tiles) that are in the trailer (aka about, ehm, 2? 3? Maybe 4?) are literally reversed.

12. The entire "LOL THE LEVL DESIGN HAERDER" thing is overdone already. If you're gonna try and make your game 'uber hard', I'd suggest that not be your main focus. Your main focus should be fun, varied, interesting, non-bland gameplay, freed of as many bugs as you can free. Of course, while designing your levels, if you really want to - try to make it hard. But don't make it fake difficulty - precision jumping, enemy spam doesn't count.

13. Camera bugs. BUG. TESTING. PLEASE. Bloody hell.

14. Pointless bonus rooms. I mean, a bonus room where you hit a switch and 2 coins pop out of thin air and fall to the ground. Enough said.

15. Star coins are way too easy. Make them more hidden. Not too hard though, but not THAT easy.

16. Unpredictable Podoboos. The bane of my life. But that's just me, right? Now when you spam them ALL OVER THE GROUND, and THEN you have the unpredictable Podoboos... That's the bane of everyone's life. Including the bane of the rules of good level design.

17. Bouncy mushroom stalks are cut off. Set the sprite stalks to 7 (because if you set it to any higher than that the stalks will disappear), and use the tileset stalks for the rest. They're in the World 7 mushroom tileset, which you seem to be using.