Well, it's demonstrable that there are quests in every zone that REQUIRE a group because they're far too tough at level--and I'm not talking only about elite quests, but things like Jasperlode in Elwynn and Frostmane Hold in Dun Morogh. Those latter two are pretty much needed if a toon is to keep levelling without a lot of hellacious mob-grinding or changing zones in order to do below-level quests. And since it's pretty much a levelling requirement that players group up for some essential-xp quests, why is it that, as you accurately put it, vanilla is not friendly to group quests? I just want to understand.
But I'm sorry that my OP brings up a topic that apparently has already been discussed to death.