Firstly, this is an excellent question and definitely needs to be addressed by the devs.
But, i would say that in Wow this is extremely difficult to achieve, especially with how the game is currently designed. In the early days of Wow 99% of the population was a noob, really we all were. So your question was moot. Alot of the fun was learning and hacking your way through the game... importantly, this was done alongside all the other players on your realm.
Now the game has next to zero player interaction out in the openworld and community doesnt exist outside of guilds. Consequently players have drifted further and further apart over Wows lifetime... all very sad really.
I think the only place in the game where players of all skill levels would play alongside each other was in the world pvp zones like Wintergrasp, Tol Barad and even Ashran. Wintergrasp was a great place to buddy up with players from the top guilds on your realm all tactically doing the tasks, and new players just rtunning round trying to work it all out. Loads of fun!!!
Sadly, this kind of content only works with distinct realms and thats been killed off with Xrealm tech
Outside of that i cannot think of any way for players of all abilities to work alongside each other.... such a shame.