If you burn through all the quests on your first class, it does get kind of grindy leveling other classes. It's a little less so if you split each starting city's side quests up among the classes in that city, but it still requires other methods of leveling. And it's not something immediately obvious, so a lot of people end up completely questless by their 2nd or 3rd class.

For the lower levels, there's also guildhests; you get the first time bonus new for each class now, so they're worth doing on every class you take up.