Originally Posted by
Hybrid
The characters must live in the world as though it's real.
Think about this: In comic books, especially ones from before the mid-'10s, you know how characters will casually reference events that happened in the past, even elsewhere in the sense that they heard of it or saw it on the news, and speak to each other like that's just one thing that happened out of many things going on? Often, this is accompanied by an asterisk and an editor's note saying something like "To know what they're talking about, read this issue!".
I've been reading up on '80s Marvel and this happens a lot. I've seen big X-Men stories get referenced offhandedly in Spider-Man comics, big Avengers stuff being talked about elsewhere, the Fantastic Four's current status quo effecting other stories in a subtle way. Then of course, they'll happen by each other, talk and remark something about the past. That's a tight connection, and a shared universe running at an optimal level.
Which brings me to the main point. A key factor in a shared universe is that it actually feels like a world that's lived in. One where people live their own lives, their own perspectives, their own struggles, and their own place on the map. They know each other, but to widely varying degrees, and they aren't all buddy buddy or enemies all the time.
For example, in the Spider-Man comics, the Avengers are just a famous team off in the distance and divorced from his own life. The Fantastic Four are closer to him, but they're more like the rich relatives you might be on good terms with, and visit every now and then: not core to your own life story. The Avengers also see the X-Men distantly as an outcast vigilante group off to their own troubles, and the X-Men view the Avengers in the distance as celebrities. It's why in those superhero comics, when they do meet, the reactions can run the gamut of "Nice to see you, friend!" to "You're so overrated/You're too small fry", "God, I hate you", "Uh, do I know you?" and "Nice to meet for the first time!".
In modern comics, I don't feel that. Everyone jumps between "I want to kill you because it's an event" or "You're so awesome because the company is trying to shill you". Little middle ground. Going beyond that, many shared universe attempt falter partly due to this reason. The world doesn't actually feel lived in, and thus it doesn't feel real, thus it's hard to get invested in it.
So I think if you're going to try to make a true shared universe, it has to feel like one where people, actual people, live in it. It's not just a platform for one story after another. That's why some worlds just weren't meant to be shared universes, because they were made with a single protagonist in mind (like Harry Potter). Others will completely half-ass it hoping to make money, but not consider the actual values that a shared universe has in story potential. There's a reason why the concept of a shared universe has existed for a long time, and it wasn't because they wanted to make blockbusters.
What do you guys think? There might be more to add and discuss to this. It's interesting stuff if you ask me, and it's why the shared universe is such a coveted thing that few actually get right. If you have anything to say, feel free to do so.