There are some things about CoH/CoV that spoiled me rotten. So much so that I don't seem to be able to get into new MMO's. The key thing, I think, was the ability to join up with your friends and play with them whatever they were doing as long as you've got a hero and a villain set up. (Well, almost whatever they were doing.)
Two systems were key to that. The first was the sidekick/reverse-sidekick system. By the end days of the game, everybody was automatically scaled to the level of the team leader. Higher level players had their powers scaled back while lower level heroes were scaled up. The scaling wasn't exact so lower level folks scaled up weren't as strong as a "natural" high level player but it was close enough that they could carry their weight.
The second was the instancing mission system. When you did a mission, it was in your team's own little instance. As you moved along and enemies spawned, the size and power of the enemies was set according to how many people were in your group. Since this happened at spawn time, adding or removing people from the group mid-mission was accounted for fairly well.
So, on a typical night, the east coast folks in your game group(s) would log in first and start doing something or other. As the night progressed, other folks would log in and join up while others logged out for the night. Eventually the west coast folks close out. It doesn't matter when you show up. You probably want to stay through the whole mission but it's not a problem if you decide to leave half way through.
Another important aspect was the variety of characters with their secondary power sets. Almost all teams are viable. You don't HAVE to have a healer and you don't HAVE to have a tank. A lot of people complained about this because they didn't have a well defined role to play in a team but that really wasn't true. Your character has a role to play all right, you just have to pay attention and THINK about the situation because that role is likely to change depending on who's on your side and sometimes even as the individual battles progress.
So you end up with a game where you can play with just about anyone and you can pick just about any character (as long as you aren't mixing heroes and villains - though sometimes even that was possible) regardless of level or class.
The only serious exception to this ability to just drop in and drop out as you like was the Task Force missions. These were a series of exceptional missions that would describe a pretty long story. Once those started, no new players could be added to the team. Players that had to leave for a bit (voluntarily or via a program crash) would fine themselves still on the team as soon as the logged back in.
These days, though, the new MMOs seem to divide players up into two groups: the team players and the story players. If you want to play with your friends then you need to do these two bits of content over and over again. If you want an actual story and a lot of content then you'll need to play a single player game minus the immersion. The City Of games let me do both at the same time.
Guild Wars 2 looked like it would give me some of that but, at least when I played it, the sidekick system went only one direction: down. A high level can travel back and play with a low level character but the reverse couldn't be done. That means a group of friends needs to play at the lowest person's level every night. Get tired of your current character and want to make a new one? Well, you might want to find some new friends. Want to start playing Guild Wars 2 with somebody who's been playing the game a long time? You're asking a lot.
But at least GW2 did something. Secret Wars and Star Wars: the Old Republic didn't even try. I don't expect Elder Scrolls to have anything like that, either.