I agree with class quests being a good way to teach the players various mechanics. I have not done the monk quests so I can't comment on that.
I already mentioned the optional elite mobs on the previous page. I think that those could serve as one form of challenge at low levels. For example, Elwynn Forest could have a rare bear roaming around. It could have a frontal cone attack with an animation that you need to move out of, and on the first time killing it you would get an "Experienced Bear Hunter" buff that grants you +10% experience within Elwynn Forest for one hour.
Simple things like that could bring various new mechanics to the low levels in an interesting and soloable way, while also keeping them completely optional.