IMHO there needs to be new side quests that unlock when starting another class. All the quests you did leveling your first class are still considered done, but each new class start on a character unlocks a different set of side quests in each area. Not only would that help players level the second (or more) class, but it also keeps interest going discovering more lore and story.

Repeatable quests get boring after awhile, but more story and lore can make going through the same areas more interesting.