Most MMOs make it difficult to play with your friends. Levels are a common culprit, as are character- (not account-) specific friends lists, but servers are today’s topic (and City of Heroes solved both those problems anyway). My friends play World of Warcraft on however many servers, and I can pick one on which to spend two months leveling to catch up. We have never been able to get everyone together on the same server for any game but EVE, except when we had so few people playing that we could not field a full group anyway. Whatever else you may say against the Champions model, it avoids this. There may be fifteen copies of that zone you are in, but you and your friend can meet in the same one no matter where you started.
It is a hard thing to make someone choose which 99% of the population to wall himself off from before making his first character.
Another virtue is the inherent scaling. Games have this problem across their lifecycle: how can you accommodate both early crowding and the later population shift? You do not want The Shire clogged with 500 hobbits at once, but you want new hobbits to be able to play once the horde is level 50, and then you want the level 50 experience to remain fun after the horde that sat there for nine months moves on. What about that group content?
In the early days of City of Heroes, you might have seen a dozen copies of each low-level zone as additional instances spawned. Champions Online takes the next step by eliminating the top-level server. Each zone has a lower population cap, so it is easier to have the “right” number active in it, and more instances appear as the incoming population expands.
There may not be a shared world, but you always have the right number of shared playgrounds.