Having to do old and possibly obsolete content to gain access to the new is a bad design that most MMOs have steared away from recently.

If anything there should be intressting questlines leading up to dungeones and unlocking similar content, and once quests are finished you will be able to enter.
Could be used to help build up context and lore for the location your going, redoing old content just makes it feel more of a chore and makes it harder for newer plaeyrs to catch up.