Very very interesting topic! There are many things discussed here, not sure if I collected every thought, but here some comments.
First, thanks to NaCl for posing that questions. I think PixelArt can defined as many things, a medium, a style but maybe as a philosophy too. I've read what wikipedia states about media in arts and I've got this:
In the arts, media (plural of medium) are the materials and techniques used by an artist to produce a work.
Then the example for a drawing:
In drawing, "media" refers to the type of held dry tool used and the base onto which it is transferred. The "held dry tool" normally means a pencil, or stick medium, referred to as a "crayon". Small particles of broken-off stick medium are transferred to a base or plane of production on which the artwork is produced. A typical base is paper, but canvas and other surfaces can also be used.
If I want to do something similar to what PixelArt is
to me I would post something like:
In PixelArt, the artist works on a canvas with an ordered raster built upon a specific quantum (undividable part, similar like the old idea of atoms). These quanta (plural of quantum) might have different forms, sizes or colors, but are never allowed to break the ordered raster. The artist doing PixelArt is concious about the ordered raster and the available quanta and tries either to obscure or highlight the ordered raster or a combination of it.
I just made this definition up, so it might have flaws, but let me make some examples:
- First, what we consider normal PixelArt in Computers, for example a Sprite. We have a ordered raster (in modern displays squared pixels [consisting of three subpixels, not important because the subpixels are not conciously altered, except for real subpixel work]) and quantum (palette, for example fixed 16 colors or 8 colors out of (128x128x128) colors) - valid PixelArt concerning my definition
- If we look at indexpainting, we are more on a gray level, but it's not PixelArt in my definition. We have a ordered raster and a given quantum (mostly straight ramps with different hues). But some tools ignore the grid completely and while painting the artist pays mostly no attention to the grid. - NO valid PixelArt concerning my definition
As you can see, I think the raster is more important in pixelart than the choosing of the specific quantum (in computer pixelart the pixelcolor). I don't have a problem when you use brushes which cycles the index of the colors, because it doesn't break the grid. Other tools like doge&burn, smudge break the grid. Therefore IndexPainting is in most cases no valid PixelArt. Please notice, I only look at the part of IndexPainting, the cleanup part with nice tools is PixelArt! - HighRes Painting is not PixelArt, even when it has indexed colors / fixed palette, because the painter pays no attention to the grid.
- Making a picture with mosaics is considered pixelart by me, because you have a fixed grid and a specific quantum (the tiles. They might even have different forms like squares, triangles or spheres).
- Making pictures with multicolored stones on the ground is mostly not valid pixelart, because you have no grid you could pay attention you. If you make yourself a grid it becomes like mosaic, a valid pixelart
- Sand mandala has a quantum (grain of sand), but it has no raster.
This is only as far as pixelart is concerned as a medium. In Helm's case, the part about the highres painting and downscaling is obviously not pixelart (I know, nobody stated that) and the last part about the cleanup is valid pixelart. The outcome is therefore valid pixelart (at least at the cleanuped parts). A blank canvas or a random canvas or a downscaled painting is for the definition the same. Every quantum which has been intentionally dealed with is pixelart. In a blank canvas you maybe leave some pixels intentionally white, therefore they are "altered by your mind" - they have content. In a random canvas or downscaled painting it's the same, it's maybe just harder to spot which pixels have "content" or not. Some pixels might even be correct even if they were never altered.
That's about the boring part.
One very interesting thing Helm said was "I'd never have pixelled hair strands like these if I started with a pixel doodle base for example". I claim that if Helm wouldn't have painted this image first in highres, the outcome would be certainly different!
Please let me elaborate: To me, making pictures should be a form of communication, you want so say something. Helm wants to say something, something which has a "infinite resolution" and that picture contains a head with a very faint tilt. In highres he is able to draw that. I just looked through Helm's pixeljoint gallery and I'm sure, if he had pixeled that image from scratch in the native resolution, the wouldn't have that faint tilt, it would be more extreme (only my opinion!).
Well, I can't say if you should have a fixed idea and then choose a medium or workflow or if you should be inspired by the tools you use or a mixture of it and I think in the end it's not even important (as far as you are honestly expressing yourself - bruce lee

).
Please let me make a very unpopular statement: There are not enough original downscaled paintings which then are cleaned up! Why is that so? There's always that talk about Styles, Final Fantasy Sprite, Street Fighter Sprites - there's no content which is communicated, only shadows. I think it's easy to adapt to a style on a pixel level and create nice looking pixelart. That's how the pixelation forum started and how many artists got their job in the gaming biz. There's a reason why there are not enough downscaled and cleanuped paintings and so many "not very good" pixelartists. They lack in a higher resolution idea/picture. That's why it's also important to have strong traditional drawing and painting skills. It's way harder to draw something highres - pixeling is only a handful of techniques and experience. Many might think that downscaling an original painting is like cheating, maybe because it's so rarely seen, maybe because they don't understand it (it looks too CG-like for example) or because they simply are envious (as I said, cheating). But I do think that the workflow of Helm, painting big- downscaling and refining would make more good pixelartists which even satisfies the pixelpurists.
In the end I don't really care. I think it's fair if you have an idea and you use the tools to communicate it the best way possible. Maybe it's a highres painting or a pixeldoodle. In the end, pixelart is more a philosophical thing to me. Creating the most out of the restrictions, limit yourself. I think Helm somewhere made the example of a GO-game. Pixelart can sometimes even be something like meditation.
Just ask yourself (while critisizing), are you ranting about some arbitrary constructs or are you really seeing what some fellow human wanted to express?