Hey so it's been a few days now, so I'm not sure how much you'd still care for some feedback... But I just happened to stumble upon this thread and I gave this recent version a try without looking at the dates, whoops.
I think the design is a lot on the simplistic side, but that's excusable since it's still the first world. The first two levels are a good example of that since they're very linear. The second one in specific ressembles me a lot of YI2; you could at least give it a palette or background change, so it doesn't look like a blatant edit at first glance. The other two levels are fairly more interesting however, so that's a great thing.
Speaking of palettes, I like the background colors you created, especially the pink-ish mountains in the extra level. I'm not digging the foreground of the first level that much though; I don't know exactly why, but the grass is a tad too dark for my tastes. If you want a tip, you could try changing it from the original color in hue instead of luminosity (which means moving it to the right/left in the color picker - I personally like green that's a bit towards yellow for grass, but that's your choice!)
Guess that's it for now - good luck!