/jp/'s shitposting is closer to our brand (which is quite close to 2005 era /b/), while /a/'s is closer to the new age imagereddit shitposting common to the rest of the main imageboards.
What that basically means is that /a/ shitposts by having a deluge of completely worthless shit, but isn't interesting in the slightest, or simple garbage that gets repeated simply because it is annoying. Example: '>implying, image macros, and other shit that would have been funny in middle school. /prog/'s and, to a lesser much extent, /jp/'s, shitposting consist of having a few different shitpost methods with at least some thought put into their creation (for example, the `How are breasts attractive' thread, which is completely off topic and ridiculous, but is well written and somewhat interesting none-the-less). Ideas are formed on a questionable premise, then expanded upon until it reaches it's absurd logical conclusion. It may also take the form of making a huge deal over a minor detail, or assuming things that are false and writing a long critique based upon that, or making provably impossible demands. Contrary to Standard 4chan Shitpost Model (S4SM), where the intent is to annoy, the intent of our shitposting is to entertain, provoke discussion, or simply as an exercise in writing.
Since the fall of Usenet, w4c style shitposting is currently considered some of the best in the world. Even the great usenet trolls have migrated to /prog/ to bask in the glory of our shitposting. The UN is currently trying to classify /prog/ as a world heritage site, but their action may come too late. W4c shitposting is currently an endangered species, due to invasion by the imagereddits, a dwindling population caused by suicide and incarceration, and moderator action. Some have speculated that moot is trying to kill all forms of shitposting that do not conform to Project Canv.as. So far, the best defense available is to derail stupid imagereddit threads with kopipe and JEWS, as well as giving harmful advice to homework threads. It is considered rude to call a w4c style shitpost in conformance with board standards bad.
It is thought by leading scholars that the reason for the Shitposting Schism was the rapid growth of /b/ due to the media exposure of Chanology, when an influx of underage Digg users caused 4chan to explode. Others claim that the shift happened even before that, during the first influx newfags during 2006. This school of thought holds that the first influx gave rise to the loutish behavior of `raiding' and other bullshit, which was a direct cause of the second major wave. Once /b/ was populated almost completely with underage children, the shitposting culture changed to it's modern form, where it is a competition on who can be the most annoying. The cancerous population then colonized nearly every other board on 4chan that could possibly appeal to it's socially well-adjusted population. Mentifex, an expert in the field, built the Artificial Unintellegence ANDRU to model the thought process of the imageredditors. Despite a few setbacks, the project has largely been a major success.
Since world4ch was not linked to on most boards, and since the average imageredditor ignores anything that isn't a reaction image anyway, the area was left mostly unharmed by the change of shitposting. It remained much more subtle, and resembled machine generated spam less than the imageboard brand. Since /jp/ was at one point full of autistic sexually deviants (a natural pairing for /prog/riders) and were far more computer literate than boards such as /g/ or /b/, they were naturally drawn to programming; /prog/ and /lounge/ (which had a shitposting style slightly different than /prog/, until two retarded children took over the board) was an obvious destination. Since it still wasn't enough of a secret club, they moved off site to places like 4-ch for a few days before returning to /jp/ with a more cultured outlook on shitposting.