One of the fun and fascinating things about pursuing old comics is discovering all those smaller-end, short-lived comic companies, particularly around the 80s and 90s, that tried their hand at the whole "shared continuity" universes. For one reason or another, they just didn't last. On the one hand, it sucks when the stories being told were pretty good quality, and the meta-narrative was pretty intriguing, on the other hand, it does at least make it easier to catch up on an old company when you know going in that there are a finite number of titles to catch up on. And even the kind of lame ones, when you can take them as a whole, you can appreciate the work that went into making a brand new world.
It's also surprising to learn just how many separate comic book companies, and thus separate universes, there were, before they all started gobbling each other up. Malibu alone sucked in a hefty number of properties, creating a whole multiverse to contain them all, before Marvel bought them out. And of course, DC had been snatching up smaller companies left and right for decades.
Of the ones I've gone back to, I think CrossGen, Malibu's Ultraverse, and the original Valiant universe has a lot of good stuff in them. Currently, I'm catching up with Malibu's Genesis Universe. It's... not as good as the Ultraverse, let's just say, but its a good example of this kind of project.
Any Dead Universes you're into? Any you're still collecting or interested in? What's your favorite or least favorite?