Tom introduced me to a new feature yesterday which is so awesome it's amazing. I'm honestly astonished that World of Warcraft hasn't added some variant of this feature. (I was first introduced to this style of feature 6 years ago in City of Villains.) The basic idea is the party leader can choose a person in their party to synch up with. Then every character in the party who is higher level than the targeted person gets leveled down to the same level as the target. They lose any abilities learned above that level. They have their skills capped as if they were actually that level.
The party then gets to gain levels as though the characters were actually the lower level. This experience gain gets applied to your 'real' level, but you gain it by fighting mobs appropriate to the target. FFXI has a 'merit point' system so people at max level can keep gaining experience to buy marginal stat increases. If someone is max level and gets synched up with someone lower they still gain merit points! So there's actually an incentive for max level characters to help out their buddies. Not only is an incentive; it's actually possible. In World of Warcraft if I had a friend start fresh now they'd have no actual way to play with my main character in any meaningful way for months. With the synch system we can have actual challenges fighting level appropriate mobs the whole way up.
When I used to play FFXI there were level down effects when you fought level capped bosses. However, when that happened, you couldn't gain experience and your gear fell off if you couldn't use it at your new level. The gear falling off part has, thankfully, changed too. Now if you could equip your gear with your real level and can't with your new level your gear actually gets deleveled too. The stat boosts on your gear get decreased proportionally in a way that isn't 100% clear. I saw a post from Square saying it would be worse than optimal appropriate level gear but that it's better than the alternative which is naked. It does mean you don't need to carry around a set of level 20 gear if you want to fight the level 20 bosses. If a specific piece is awesome you can still store it, but it frees up a lot of bank space.
There are a few restrictions, most notably that the target has to be at least level 10 and you all need to stay within range of the target to gain experience, but it seems really incredible.