Pretty good summary, Bob. I feel like a proper understanding of what happened with 90s comics would require going into the history of the direct market and Diamond, and the subculture they created. E.g., the direct market took superhero books out of the drugstores and supermarkets where most kids discovered them, and put them in specialty shops which the parents of young children weren't likely to enter, skewing the age of comic buyers into their teens and early 20s. However, this also provided an environment where Big Two books could explore the lessened restrictions of the Comics Code Authority [http://en.wikipedia.org/wiki/Bronze_Age_of_Comic_Books], and where new smaller indie publishers could float edgier new material like Cerebus and the Teenage Mutant Ninja Turtles. Those are the roots of your four pillars. But covering the Bronze Age of Comics would need its own episode, and frankly it deserves it: it's a long-neglected but extremely foundational era of North American comics.
Anachronism said:
Wasn't all bad. The '90s was the decade that saw the rise of Vertigo, which has published a lot of the best and most influential comic books of all time, in my opinion. In terms of the medium growing up and becoming actually mature rather than superficially mature - as most superhero books of the time were - the importance of Vertigo can't be overstated.
And while Image admittedly isn't a powerhouse on the level of Marvel or DC, these days it looks likely to surpass Vertigo as the place to go for interesting, original, creator-owned comic books. The Walking Dead is the obvious big name, but you also have great stuff like Saga and Age of Bronze (a personal favourite), which probably wouldn't be published by the Big Two because they don't really fit with the rest of their properties.
Seconded. Vertigo and Image gave most of the British Invasion guys (Alan Moore, Neil Gaiman, Warren Ellis, Garth Ennis, and some of their artist buddies) their first real breaks in North American comics, which eventually led to the replacement of the Superstar Artist with the Superstar Writer. Around 75% of all the comics I've ever bought came from these two imprints.