I don't like leveling scaling and never will. If you people want to meta game and power level to skip the content they don't care about and only play the content they want, why not let them. There are other ways to combat this like Albion Online/Eve Online leveling system.
Onto the issue of keeping all content relevant. Rather then being able to take any quest at any given moment, players should have to work up to being able to do the higher level quests after gaining rep with the faction/quest giver/etc. Sure higher level players will be able to blitz through the lower level quests but they will have to do them in order to gain rep with faction/quest giver to unlock the higher level and more rewarding quests.
Next is the story. If you're 10 levels higher than all the enemies you are fighting in the main story content, you wont feel any attachment to the world and by extension the story of the game.
I don't like the idea of having a main story. So all hundred thousand or so players running around are all heroes that saved the world? I'd rather have players decide their own story through player interaction rather then let the game decide what you are. While letting quest givers/factions give out the quest storylines that you can follow.
EX:
Level 1 - Quest giver starts you off with killing x amount of goblins.
Level 2 - The goblins you killed seemed to be holding notes containing orders, suggesting a hierarchy of leadership. Piece together who their leader is
Level 3 (Group recommended) - Defeat the goblin shaman in his cave of operations
Level 4 - After defeating the shaman you've learned of an attack on a village. Defend the village and stop the attack.
Level 5 (dungeons) - Defeat the other goblin subleaders before they initiate attacks on villages.
Level 6 - After defeating the goblin subleaders you've learned of a goblin king ordering the shamans to attack the surrounding villages of a big commercial town, in order to weaken their forces, logistics, and food supply.
Level 7 (Exploration/investigation) - Find out where the goblin king's base of operations is
Level 8 (raid) - Stop the goblin king