Most other MMOs i played didnt have the ability for a high level character to join a lower level team so they didnt have to balance being over powered. They would just release a new raid where your current gear wasnt good enough and had an endless cycle of collecting the current gear that was better then your old gear. They would often increase the level cap during this process as well. These MMOs are 'growing up' and often excludes under geared characters from participation in high end content.
In contrast, City of Heroes is 'growing out'. New content is being added but all of the 'gear' is obtainable by anyone and not locked behind the top end raid. And with so many players making characters that can solo the game at +4x8, there is no nerf that could possibly work at making the game too easy on current content.
Cobalt mentioned a new level of challenge is in the works and we will just have to wait till it makes it over to the test server and give it a shot.