City of Heroes has practically an embarrassment of riches in game design. I keep hearing design dilemmas in other games and thinking, “Oh, City of Heroes solved that problem years ago.” The solutions are usually heavily reliant on instancing, but how much of your WoW time are you spending in instances anyway?
Many games borrow the City of Heroes sidekick system, in which you can bring a teammate up (or down) to your level. City of Heroes has gone beyond that: everyone on the team is the same level, and all the instanced content is always the right level. It really is that simple. Set everyone to the same effective level. Set all the enemies to that level (or let the players dial up the difficulty). Done.
Some of this works due to a good design in ability acquisition. You do it once. City of Heroes trainers do not try to re-sell you Fire Bolt every five levels with a new number attached. You buy it once and it scales with your level. If you join a team that changes your effective level, your abilities scale up or down appropriately.
Not enough content in a level range? Every quest can work for every level range. Friends just joined, or you are joining a group of level-capped friends? Everyone can play with everyone else. And content scales to team size, so bring as many friends as you like.
Fun bonus thought: if Blizzard really wanted to, they could have the hit points and damage of every enemy in every instance scale to the gear score of the party.