I feel like a mathematical proof is trying to be settled in this thread.
I am more interested in the motivations behind the styles being discussed than creating a wiki.

How has hardware affected style through the history of pixel art?
-Larger palettes, sharper pixels, higher resolutions, etc.

Is the artist trying to push or even break the limits of (an older) computer (as is frequent in the demoscene), or are they looking backwards, to emulate old hardware limitations using a modern machine?

In amending my definitions I would probably remove old and new school, and replace it with a grey area dependent on hardware. Of hapiel's definitions, I think Bubblegum and Hi-Def are definitely observable trends. Demoscene, retro, hybrid, and iso (a perspective and a style) are also pretty easy to spot, though of course there is plenty of overlap between a lot of these.

when I read "Why did this have to be pixel art? Would have looked just as impressive in another medium" it gives me a headache

This.

Scene 3 is probably my favorite but the flat blue background could use a little spicing up imo. Nice work all around.

this is looking much better but the trunk isn't matching the foliage. the foliage looks much more realistic. the high contrast, saturated colors, and black outlines on the trunk make it stand out. the black lines in particular I would eliminate, and also try to make the texture conform of the contour of the tree and vary a bit instead of a random uniform texture. pushing those browns closer to gray would probably help too.

Except his example has like 8 more steps than the owl. This is a subject whose construction has been discussed pretty thoroughly in past tree threads, so repeating it would be redundant.

And searching the forums for those threads would probably be quite helpful. Briefly though, you need to break up those clumps into more natural foliage patterns. Can't understand the foliage itself, are the leaves dripping or is that moss or some weeping willow hybrid?

WIP

Original

Still lots to clean up. Feedback welcome.