Cydmab

For the record, you CAN keep "advancing" your character at level 50 for a significant period of time; there exist rare and expensive special enhancements to collect to continue upgrading your character. If your character is not being twinked, you could easily spend months collecting the resources to totally max out your character. However:

1. Any level 50 can reasonably group with any other level 50, regardless of gear. (If anything, differences in powersets matter much more)

2. There really isn't any completely new and alien content at level 50 that did not exist before level 50; as others have said, you don't even have to reach level 50 to experience most of what the game has to offer.

3. What little special level 50 content there is, virtually all of it is available to you as soon as you reach level 50. You do not have to keep "gearing up" to access more content. An interesting exception might be personal challenge activities (can I beat a mission designed for a full group of 8 by myself!) but a) the player has to kinda make up these challenges themselves and b) there are rarely special awards for doing these things and c) of course, there is no gated content. (Another exception is maybe PvP - I know PvPers who insist on having a totally maxxed out super enhancement build before participating in PvP)

Putting top priority on being able to play immediately with no arbitrary barriers between you and content while tying the experience to a persistent game world is an innovation I'd like to experience.