I as altoholic have leveled several toons since CRZ, playing on a PVE server.
My experience is mainly <lvl 15 and in outlands/northrend, since there is no real need to level "outside" from 15 to 58. And with removing the automated flight paths, they even took out the last incentive to go leveling outside of dungeons since it is pointless to walk 5-15 minutes to the next questing area when your dungeon invite is coming in half the time and turning the mobs grey.
So this in mind, I have only had bad experience with CRZ. While many of the cata redesigned quests are able to support many people doing them at once (mob sharing, or mob drops quest item on the ground), some don't. One example might be in Westfall, you have to do some eavesdropping on bandits. There are four of them, and you have to tag one of those after listening to them for about 90 seconds when they become attackable. At times, there are dozens of people standing there and competing for the mobs, not only leveling chars, but also lvl 90s who want to complete their achievement, all spamming AoE. This often means that one get the tag of the four, "wasting" three other possible slots. That means you have to wait about 30s for the respawn of the mobs, plus the RP time, and the people waiting are getting more instead of less. I often decide to end questing at that point, and start grinding mobs to reach level 15.
Sure, this is a typical example where grouping up would help - since you are not yet in a lfd queue. But even if LFD queues would not drop when you join a group, grouping up or even talking to other players is something that simply has no place anymore in wow. I don't deny that group quests in vanilla caused many social contacts, I met several of my still-ingame friends while leveling.
But that was another time. It took a time to get your group together while leveling, especially because quite in the opposite to CRZ the areas were not swarming with people. And this caused direct interaction with other people. Then we would talk about quests, if I know how to find this, were can I turn in that. People had questions about the game, and there were no in-game addons or websites where you could find every answer you could have in the game. Today if someone asks me where to find something - and if someone does, you can be sure he is new to the game, which is not very common anymore - I simply tell him to look it up at wowhead. Not because I want to behave like a asshole or a teacher, but simply because this is the most efficient way to find it. Before I write several paragraphs to describe where he should go, he will find a map with an X on it.
So, what I want to say is that CRZ only creates the illusion of a world full of players who interact. They don't, because they don't have to. And I am fairly certain there is no way of designing the game so that it feels like in old times. Even if they would create new group quests that would require finding a suitable group (not like the pre-cata group quests that you could roflstomp with every heirloom char), I bet 90% of people levelling would simply skip those. You would find dozens of skeletons because people would try them alone.
The truth in my eyes is quite obvious: Leveling is not about exploring or discovering, but about efficiency. "Socializing", even writing "hi" at the start of a dungeon, does not fit in there.
So I don't see any positive aspects about CRZ, only how it affects me negativly with more competition on mobs and nodes.