You're understanding pretty well. What is pixel art ? What is art ? Almost the same question. You have a lot of possibilities, a lot of understandings. Pixelation & Pixeljoint are on the conservative side of things, while some others places like deviantart are a lot more progressist, with effects that don't look like pixel art (smooth lighting, etc…)…
Well, what I find interesting is what people choose to pixel. Now, I see a lot of talented artists who create amazing, deep, provoking art with photoshop, oil paintings, charcoal sketches, etc. And they restrict themselves to doing NES mockups and nostalgic Pacman wallpapers when working with pixel art. Other people create pixel art that looks like it was made in photoshop, and seem to enjoy defeating the restrictions of pixel art, rather than using them to create a unique look. I've seen comments on PixelJoint like "Why did you draw this with pixel art?"
Looking at the oldest pieces in the PJ gallery, it looks like this division was there from the very start.
I suggest you read Syosa's interview to get his experience on japanese pixelling community :
http://www.pixeljoint.com/2009/10/03/2938/Pixel_Artist_-_Syosa.htm
I've read it, but I didn't find it very informative. It was 5 years ago and you quickly run into broken links. The interview didn't really show me a thriving Japanese pixel art community, but absence of evidence is not evidence of absence, as they say.
On sites like pixeljoint you can see an obvious jump in quality between the older pieces and the newer stuff submitted... but can you really say the quality of pixel art in general has gone up? Nah. Pixeljoint as a site grew and attracted more artists. There's still pixel art probably as old as I am that is better than a lot of stuff you might find people making today. It just depends on the artist, not the community as a whole. Though I suppose as time has gone on with less restrictions and refining the genre, artists are capable of pulling off much more amazing pieces. Still, some old art from people like Henk Nieborg still looks amazing in my eyes.
There definitely seems to be a jump from 2005 to 2008-2009. But I agree with you, some of the best oldest pieces on PixelJoint are just as good as the stuff people are doing today. Maybe we're seeing people experiment a little bit more with the somewhat unique possibilities that comes from pixel art, such as
this or
this. At least, that's very exciting to me, in addition to keep the old art form alive.
Pixelation and Pixel joint have fairly less to do with games or game design as a community.
Both are art communities so to say, but there are other communities, like game Maker forums, Byond etc. where people also use pixel art - and they have their own understanding of the art form, and this understanding develops completely unrelated to Pixel joint or Pixelations "rules of pixelart".
If you are searching for another huge community, go to Deviantart and look how the people there are "understanding" pixel art and what kind of art the artists there create. Deviantart is for sure bigger, most hardcore pixel people however hanging around at Pixelation or Pixeljoint - and I suppose you are looking for a similar comm - there is nothing comparable I know of.
That's interesting about game maker forums. I'm not looking for a substitute for Pixelation or Pixeljoint, I'm just curious to know if it's out there. Do I have a good understanding of the state of pixel art, or is there some huge underground community in Turkmenistan where people are drawing wonderful pixel art on their C64 and spray painting it on abandoned factories with cardboard templates? And Japan, where the power of nostalgia is perhaps stronger than anywhere else in the world, is almost like a separate world to me, because there are so few Japanese people who come to western websites like this and so few Japanese websites where people primarily write in English, I think. It's like if you have some other niche hobby, like zen tea drinking, you start wondering about the global zen tea drinking community and whether you're a part of something vast, or if you're preserving a small and dying art.