I think the "pixelart feel" also has a lot to do with a certain amount of colors on a certain level of size.
I think color count is pretty key as well. More precisely, how the palette affects the technique, and vice versa. With a low amount of colors, you'll probably be doing more dithering, which exposes the pixel grid. With a large palette, you're delving more into AA and gradients, which is a more subtle value shift, so the grid and pixel relationships are less apparent. Even a relatively small palette could elicit a "I can't believe it's pixel art!", if you've got 10-20 shades of the same color, because that one long ramp allows you to create such a smooth image.
IMO that falls into the same 'meaningful decisions' theme that some people like to use when talking about this. That is, a gradient just looks like a gradient, if each shade doesn't have 'individuality'.
I agree, the individual identity created by very blatant relationships between clusters is key to the "pixel art look." Because I tend to think the borders between clusters are more important than the total number of colors, I did a little test where I tried to maintain the general cluster identities while throwing in tons of colors via gradual gradients:

Or another definition of resource appeared: development resources. The ability to make indie development effectively possible.
This is an important limiting factor that often gets eclipsed by technical concerns. It not only leads to very pixelly, low-res, faux 8-bit art, it also leads to gradients and glows (which I'm fine with in moderation, so long as the cluster relationships beneath are still bold).
The middle has the greatest number of reasons for why you would do pixel art for most applications. Going to the left noise, the reasons get fewer for why you would do any art like that. It sure can look very fascinating as an effect, and there can still be a very good reason for why you do it in some case, but in the greater scheme of applications, it can only be a niche. At its best it is a gimmick, at its worst its useless.
In general, I agree that the sophistication and applications of the right end of the spectrum (blur) are much greater than the left end (noise). However, in regards to pixel art, I think there are reasons to choose a slightly looser approach, namely the expressionist/impressionist quality it allows for. I think this heightens the creepy factor in Uno Moralez's work.
Most cases of application for pixel art require the clarity of the middle. It's what works best for most people, it's the norm for good reasons. Pixel art has the strongest identity and greatest use in the narrow part. Most people that want to create pixel art, will want to know how to create that. The defining quality of that organically rises from these greater interests.
I agree. I think that the "no single pixels" pure-cluster art is the dead center of the spectrum, the sharpest point, neither noisy nor blurry. But I think there are also advantages to buttery-smooth AA and even random dither. Some think that smooth pixel art might as well be created in another medium, but as AI said earlier, it "fits" the resolution in a way that other approaches cannot.
Eastward and Superbrothers are both very much traditional pixel games, despite having non-pixel effects and lightning on top. Dan Fessler's squirrel mockup is a classic example as well, and should be somewhere in the middle I think (...) Index painting would certainly be on the far right side, borderline with digital painting.
By the way, where does this sample with red mountains and blue forest comes from?
Good point. I originally made the chart thinking about the types of pixel art allowed at PJ, which is more likely to include index-painting than hybrid stuff. But I think the examples I chose (along with the Slain! game you mention) don't belong on the chart in any linear way, since they combine approaches from different parts of the spectrum. Maybe this is better:

The image with red mountains is by an artist who goes by cutlaska or captain-carrion (often just "carrion", but that name is already taken!).
"why pixel art?"(...)for me it would be crispness or sharpness.
I agree, and building on that:
(...)it's really about whether the automated bits are unobtrusive. Things get messier for me when elements like glows, gradients, and shadows are integrated more deeply. I'm not a fan of detailed pixel work being disrupted, you often get muddy colours and unwarranted attention drawn to higher-resolution, unnaturally smooth elements.
These are all good points. I'm fine with the soft glows in the bar scene on the right side of the chart because they don't obscure the intentional pixel placement below. It's just a thin film, the core of the medium is still there. This might be complicated if the underlying pixel art is more smooth or complex, but the effects often mesh well with simpler styles.
As for me, I have to ask myself "What distinguishes pixel art from other forms of digital art?" I think the answer is pixel-pushing, the act of moving individual pixels. The amount of pixel-pushing required for a work to be considered "pixel art" is debatable, but I think everyone agrees that evidence of pixel-pushing should form the basis of the work's aesthetic if it is to be called pixel art. The introduction of "NPA" techniques complicates our definition, but I think it's too dogmatic to say that every instance of these techniques disqualifies a work from being pixel art.
I think the pixel art aesthetic is maintained if:
transparent lighting or atmospheric effects are used over visibly pushed-pixels
stray pixels are used as an expressive or impressionistic dither
And lessened if:
the borders of pixel clusters become blurred and obscured
Individual pixel placement is largely incidental