The sidekick system
This.
SWTOR does something loosely similar to sidekicking, but it only occurs in PvP areas. In a PvP area, you're affected by 'bolster', which essentially scales your character to level 49 if you're lower than that. Because the game is gear-based, there are still some places where it's not working well, like the 'naked bolster', which grants a character a lower degree of bonus if they don't have a particular category of gear (i.e., lower-level characters who can't get anything for their head slot). So it functions more like the way CoH's PvP zones flattened everyone to the same level while they were in the zone. Unfortunately, because this only works in PvP areas, in all of the content where you are expected to team (the 'Heroic 2', 'Heroic 4+', 'Area 2', and 'Area 4' missions, or world-boss fights), if you have a level gap larger than about six levels, the lower-level characters start losing XP.
As a result, there's no good way for you to bring in friends and play with them after you've been playing the SWTOR for a while; if you play your high-level characters with their low-level ones on PvE missions, they get squat for XP, so you're stuck creating new characters just to be able to play with them at a comparable level. This discourages you from bringing in your friends, where
City of Heroes would let you get friends set up with the game, let them get through the tutorial, and then you'd be able to take
any of your characters and team up with them to do their missions or yours without one or the other of you getting jerked over by the level difference.
Sidekicking is the one feature of CoH that made it possible for new players to get drawn into the established play of characters that had been around for years, exposing them to a wider view of the game instead of being stuck with the much smaller group of characters at and near their level. The "we're all superheroes together" result, I think, helped foster a community that I don't see any other existing MMO matching.