Well ideally, in 2.0 we will have a LFG content finder system, where finding people who need to complete a task can find others in the same situation (along with "cross server" content, which if storyline is largely instanced could easily be done). Plus, no one will have done any storyline come 2.0, so there should be a huge amount of people at all times looking to do these quests.

And considering yoshi wants a lot of leveling to be done with quests, the rewards for doing them will be pretty great, which should be enough of an incentive to do the quests.