I think this piece at its current state simply does not have the standards of what most would think a pixel work should have. Including the common rules here on PJ.
As others have stated, I would advise making a 64-128 colors version of this work. 
I can see a big potential in this piece if you give it time and effort, keep it up!

Yep, the core of the issue was that I tried to cling tightly to the original and lacked the will and abilities to transfer it to another medium. The piece I am currently working is original and uses a very limited palette, so maybe I can hit a healthy middle ground after that one :P

It really depends what you want to copy (keep). Keeping subtle color ranges faithful to the original condemned you to a very large and complex palette.

A monochrome rendition would have been more manageable (or near-monochrome : two color ranges, one hot and one cold)

Very often, copies/transfers are not meant to be viewed side by side with the original, so they have the possibility to explore very different color spaces : Example with a transfer to amstrad which has a very small and unforgiving palette : http://cpc.sylvestre.org/musee/mode0_demoecrans/batman_foreverf_dadman.gif

Original : http://photobucket.com/gallery/user/jesselopez1984/media/bWVkaWFJZDoyNzk5MzU=/?ref=

Very beautiful

I was curious so I went ahead and downloaded your piece and used a simple posterize tool to mathematically reduce the color count. Here's the result of that -> https://i.gyazo.com/cf9749da0a81668bf9bbfddb946a15e1.png

Then I took that image into the Image Specs tool that PJ has under the Features tab at the top of the site bar and fed it through - that image I sent has about 140 colors. Then I took a closer look at the colors information, which shows me a list of every color used, the percentage of pixels that that color is used in, and also the specific pixels where it's used on a nice little preview version of the piece (I highly recommend checking it out yourself, just plug the screenshot I sent into the tool). 

Of the 141 colors, more than 60% are only used in 0.1% or less of the total piece. Obviously, this is somewhat skewed with the size of the piece, but regardless, that means that each of those colors is being used for a very brief purpose in a very small, usually localized section of the piece, probably as a result of you color-picking based on attempting to mimic the reference, rather than re-using a palette of sorts. 

Now it's not bad art form to use all of the colors you have access to, and I hope you don't think I dislike the artwork (though I'm only willing to give so much credit since it's an interpreted version of an original piece, though I think you understand what I mean), but pixel art is very heavily reliant on a sense of control and cohesiveness of the piece. Since you're able to control every individual pixel, the challenge then becomes, "how effectively and crisply can I communicate my art?". Pixel art is the only digital medium where you have absolute control over color and shade (as compared to using a brush tool in Photoshop) and pixel per pixel design and technique, and so using it in the same way that you would digital painting (high color count, lack of clean, hard edges) is both limiting yourself (because if you were using a brush tool it'd be much easier to blend), and missing the point of the pixel per pixel control. 

I know what you mean though, hopping from any art form over to pixel art and vice versa is incredibly fuckin rough. I am absolutely terrible at drawing/digital painting because I'm so used to fine-tuned control, that I get overwhelmed by brushes and ink that bleeds and rubbing graphite all over the paper AAA. 

Talking through PJ comments isn't super intuitive, so if you'd like to get some more real-time help or just chat with other pixel artists (and me), I highly encourage you to join the unofficial discord server for the site --> https://discord.gg/TBEkUEU

Thanks for reading all that :3 I do want to make clear that, at the end of the day, this is art, and how you do it is up to you. I'm really not normally this much of a technical artist dude, and I break a lot of the rules myself at times, but I think it's important to know what the rules are so you can actively say "fuck the rules", rather than just be stumbling around confused. 

Thank you for your constructive feedback and hints / examples on how to improve. I still feel those pictures have a color range way narrower than the one I tried to replicate, but I suppose something like 128 colors without losing much of the impression might have been possible...
Sorry, I feel like I'm bargaining on a bazaar and being too defensive here :P

One thing I noticed again and again when working on this piece was that I overestimated how jarring the contrast between two colors is because I focussed on it and was zoomed in (even after realising this and trying to keep a constant eye on the preview window, I often was not satisfied and introduced another middle tone).

More creative use of dithering and combining colors to "create a third" is definitely a concept that is still foreign to me (have started doing pixel art on the side at the beginning of this year and only did some pencil drawing before).

You've got a massive range of colors of very similar color value/brightness that can absolutely be used elsewhere to replicate the color without actually using a new one, mostly through precise dithering, as well as a significant number of per-color shades, so when you have, say, 4-8 tints/shades of a particular color, and 3 colors that are already very similar to that original color, the color count multiplies quickly. Additionally, there are some parts of the piece that have extremely subtle hue differences that are 50% probably from the original compression of your reference image and 50% a hue from a smooth paintbrush tool that you won't be using in pixel art. 

https://i.gyazo.com/8c888981eacc5c28c15aad5251cff398.png [ this blue is one of these subtle hues I'm referring to ]

As for how to do things like this in a lower color count, it's obviously extremely rough and time consuming, and I'm not attesting to being able to do it myself, BUT, if you want examples of color count/gradient mastery, take a look at Elk's profile or ctrlv the following two links for two of their best pieces, which have a very similar style to what you're trying to replicate here, and in both instances, less than 65 colors.

http://pixeljoint.com/pixelart/77504.htm
http://pixeljoint.com/pixelart/61838.htm

For a piece that uses even less colors, but has a larger hue range (similar to what you have here), take a look at: 

http://pixeljoint.com/pixelart/69512.htm

It is somewhat more simplified in the sense of gradients, but I'm sure you can imagine what Elk's shading technique in combination with those colors might amount to. Good luck!

I know where you're coming from, but I just wanted to get as close as possbile to the original motive. When working on all those gradients between the dozens of colors especially the sky and ground contain, I just gave up on palette control. And btw, its 286 not 386, no?
Its probably possible to get it to 256 colors just by cleaning up and without any noticable differences, but I was drained by working on it for so long and really didnt see a point, as that's still a shitload of colors.
I don't think it would look any like this If I somehow kept it at 64 colors.

Anyway, I learned my lesson and will not attempt to "copy" such a motive in pixel art  again. I'm interested how others handle their palette when working from a original with such properties. Do they simplify the whole image? Do they just forgo the claim of sticking to it and do more of an interpretation?