Guild Wars 2 does this. It doesn't matter which avatar my girlfriend or I feels like playing, we can group and have fun in any zone, for any event. After experiencing this, I don't understand why any MMO would not incorporate dynamic leveling into its design. I really don't understand why this isn't a standard mechanic for MMOs that want to encourage grouping.
For those unfamiliar with dynamic leveling, it's a mechanic that scales character's levels down to the areas they occupy. Level fifty-fives would scale to level ten or less on Korriban. Granted, with gear and abilities, they would still be pretty uber. The mobs and quests would still grant experience and they could help their low-level friends and not ruin their experience. No longer would I be able to return to Dromund Kaas and farm the greyed heroics for planetary commendations, while lower levels, for whom the content was created, wait for a group.
I hate forced grouping, but for content designed for groups it seems grouping should be a requirement. By not having dynamic leveling, MMOs actually hinder players ability to find groups. Ruining a lowbie's experience gain creates a reluctance to help out.
Dynamic Leveling not only helps out grouping, but developers as well. New content doesn't have to cater to the endgamers. A new expansion or world boss or event could be any level and still offer a challenge to the fifty-fivers. New events could happen anywhere and have more meaningful participation. Level Scaling to the environment keeps the environment alive to all levels of players.
Fifty-fivers could organize ops groups on newbie worlds, teaching the less experienced the skills needed. This would certainly make many guild leaders' jobs easier. Too many are discouraged by the learning curve to join elite ops. In general, guild events would face less challenges.
SWTOR is a very anti-social game. Players are isolated by their story, their faction and their experience. Dynamic leveling might make it less so.