Characters that weren't anchored to specific campaigns, and conversely, campaigns in which the players all played multiple characters.
Back In The Day, a goodly chunk of my gaming was done at conventions. I had a group of friends scattered across three counties of Southern California, and the only times we could really get together to play were at cons. Our characters were mostly "based" in my friend Kurt's world -- though my main character had actually started in another campaign entirely, and outlived it. The notion that I had to give up Lados, Erbec, or Drakewynne because I wasn't playing in that DM's game anymore never occurred to me -- or to anyone, in those days.
Kurt would usually run a long, multi-part session in his world, where we'd band together against some ancient evil that posed a threat to the world/the universe/us personally. Kurt had an uncanny ability to handle a table full of a dozen players, many of them running two characters, more smoothly than other GMs can manage five or six. As a result, those games would almost always have "guest stars", other convention-goers who would bring their characters in for the adventure. It was fun to see new faces and new ideas.
We'd do the same ourselves, finding other open games and sitting in. It was standard procedure, back when. We each had a stable of characters of assorted power levels, and could usually find someone appropriate for the adventure. Again, it was fun, seeing how other GMs handled things, and showing off to a new audience.
Between conventions, our characters still had lives of their own. Kurt ran a weekly game with his locals, as did I; we had long-running PBM exchanges; two or three different campaigns would intertwine and feed back in and around each other. Then, every two or three months, we'd all get together and Battle Some Cosmic Menace. It didn't resemble your classic fantasy quite so much as it did the comics. We were the Justice League of Corongond, by golly -- each of us anchored our own book, and when we got together as a team, you knew something big was in the offing.
The common wisdom is that the modern approach, where characters live and die within a single campaign, where they're an integral part of their world and shaped by it, where players are expected to focus on One And Only One Player Character In A Game, is more "sophisticated", is "better role-playing", leads to "more developed and fully-realized characters".
I'm not sure about that.
I can remember more distinctive, interesting, well-developed characters from those four or five years of cheerful chaos than I can from the two decades that followed.
EDIT: In a curious bit of synchronicity, shortly after I made this post, anonymous sources pointed out an eBay auction of the Very First Player's Handbook Sold. The characters I mention above actually predate this relic (by almost a year, I think), though most of their careers were spent using this rules set.