It's the exceedingly rare print comic that can and does, like Doonesbury, have an extremely large, rotating cast of characters, and go deep into very disparate plot threads involving them without losing momentum or audience. In part that's because continuity's a lot easier when you just have to follow around, say, Spiderman, and whatever nefarious/mutated/insane industrialist/scientist/rival journalist is plotting to rob the bank/blow up town/get handsy with Mary Jane at any given moment. Sometimes tracing the adventures of one person is merely convenient, other times, that lead character is the access point for a vast interior world (see Calvin or Jeremy Duncan; as an aside, I can never read Zits without feeling like Jeremy is Calvin grown up). In the strips with larger casts, those casts and strips work because the characters grew over time and so did their universes. Alex Doonesbury would never have worked in the first place if we didn't know Mike so well. Ditto with the less sophisticated For Better or For Worse.
The web makes it much easier to expand a cast much more quickly. When readers can sit down and read through 700-odd strips in a couple of concentrated hours, you can have plots that are much more novelistic, and much bigger sets of characters. Readers can check back in, a cast page is conveniently there to pop open in another tab, etc. I realize saying all of this may seem obvious in an age when we're all used to having everything accessible, all of the time. And even the internet can't change the way we read entirely. Once you've read those archives in a novel-like chunk, we're all still stuck hitting refresh until our favorite artist posts the next day's strip, and get us another four or five panels further in plot development. Sometimes, when you're absolutely desperate to know what's going to happen, you get hit with a talking robot, or an Irish cactus, and that's okay. It teaches us patience, and unless it happens too often, it keeps us coming back. But changing the way we get introduced to comics can get us hooked more quickly, and makes us much more knowledgeable readers. And that can free up the artists to do more sophisticated storylines, and to keep around more characters for longer. So go do your part, people. Or am I going to have be stuck reading yet another strip in which Curtis woos Michelle / comments on ladies' church crowns / avoids Chutney? (I REALLY miss the glory days of The Boondocks.)