The foreground is looking too pixelated, it would be better if you made it smoother instead of just increasing the size. Some of the blocks and the pipes need black outlines.
The giant blocks are also in a different style of the normal-sized blocks. It's not really a problem, but it would look better if the were on the same style.
The OW is looking too flat, mainly the left part. Add some more variation there.
The are around 0:36 is a bit too cramped.
Is there something in the turn blocks around 1:37? If there isn't, I suggest that you remove them, because they would be pointless. This part is also lacking enemies.
Not exactly a fan of the Blue Flames Bob-Omb replacement. Sure, you explained it in the message block, but it still looks a bit weird. The redrawn vines don't really fit with the rest of the GFX.
The water at 00:40 is really random, I suggest you remove it. Making some 'connection' tiles for the forest GFX would be also nice.
The water pool at 1:02 could have something there. Maybe you could add some cheep-cheeps?
The dirt tiles at 1:18 are the same, there are two tiles there for variation. Use both. The blocks at 1:36 could be replaced with cement blocks, as those 'grass blocks' don't look that good.
The grass formation at 1:53 is really weird, maybe changing the parts that are out of the dirt to cement blocks and removing the parts inside of the dirt would look better.
2:03 could be changed to a normal ledge.
2:07 is another random water pool. The area around 2:07 also needs some decoration.
This is looking nice so far, with some nicely designed levels and a simpler style. Nice work!