OK then, it's review time!
Level design: 7.73/10
Let's be honest, you're trying to trick me into awarding a high score because of your fancy graphics, but I can see through the graphics into its actual design, and considering that it focuses more on the gameplay than the graphics. this level is very, very good. In fact, it's one of the best 8-bit SMB1 levels I've seen, and no, it's not too long. You're forced to play through it as Luigi, but this level plays to Luigi's strengths very well, with the inclusion of large jumps and long pits with little ground. As well as this, I like how you took the nonlinearity concept and threw it on its head by giving the player an option of two paths to take every time, but it doesn't affect the overall outcome of the level. However, you took this idea and really drove it to its extreme limits, as each section has got something unique about it which is largely retained to its own section and probably another one as well, with contrasting appearances ranging from an underwater part to a climb using moving platforms, and these are executed very well in their own right. Sadly though, what was stopping this from being an awesome level was this level-breaking flaw:
This screen was taken at the highest point of the jump - if the platform falls and you can't make it to the top, the jump becomes impossible, and the player will have to commit suicide. It should also be noted that the rest of section 8 is quite bad as well, not just because of this, but also because in certain spots you can't get out of the water and have to commit suicide, so lowering the platform so the player can get out of the water is recommended. However, this level is still really good, and I really had a blast playing through this level.
Graphics and music: 8.2/10
I'm not really a fan of 8-bit music in SMBX because it clashes with the 16-bit sound effects, but in this case it's been used perfectly and it fits in with the theme of the level, so I won't mind that much. As for the graphics, they're absolutely stunning, and it should be noted that this is technically an 8-bit SMB1 level as mentioned earlier, but this level feels far from it because of the graphics, which provide a really authentic digital atmosphere that almost immerses the player into the level experience. Therefore nothing really clashes with each other except for the out-of-place SMB3 star at the end of the level, which should really be recolored into the style of the level. It's as if the level was designed around the graphics used, not the other way round, and the colours of each tile dictate the theme of the section (for example, red for lava, blue for underwater). There are still a few odd graphical flaws, such as the Thwomp death effects being incorrect (but that's because one individual Thwomp has two sprites for it), but the beautiful graphical presentation outweighs these minor flaws. Never have I found a level that emphasises the motto of "it's not about the custom graphics, it's about how they are used" as much as this one. Excellent.
NPC placement: 7/10
Do you seem to hate flowers and leaves or something? Because according to the debugger, there are only mushrooms in this level as the powerups. Still, they were placed in good handy locations, so whenever I needed a powerup, it was there waiting. I suggest that you need to make the powerup range more diverse, even if it includes changing all the blocks containing mushrooms into Fire Flower and/or Super Leaf boxes. The underwater sections, as mentioned earlier, turn out to be the most difficult ones, not just for the flaw mentioned earlier, but because they are filled with Torpedo Teds that weren't implemented properly - they generate too quickly and, combined with the slowed underwater movement, adds fake difficulty that shouldn't really be there, and usually resulting in easy deaths - so decreasing the number of Torpedo Teds would be grand. Coin placements were OK, and most of the time they were factored in towards the level's design, but some Dragon Coins would be welcome considering the nature of this level. What I really liked was how each enemy was designed to fit in with the colour of the blocks, and thus the theme of the section, like the Podoboos in the red block section and the Cheep-Cheeps in the blue block section.
Overall result: 7.6/10
Now that is one level I don't see every day, and even with that in mind, it's an awesome level! Fix the Torpedo Ted problem first off, and try to style all the graphics into the 8-bit of the rest of the level. What the best thing about this level is is not just that it's good, but you can come back from not using SMBX for ages and still make a good level.