The title says it. Let me explain by showing you my new undead priest. His name is Laszarus. He is now level 7, but a few minutes ago he was level 6. He has done every single quest up to that point and killed just a few extra mobs.
On his way to Brill he encounters the Cold Hearth Manor. He gets a new quest near the road to kill some of the undead.
He succeeds. Here's the video. As you can see, he does fairly well. (Look at the health bars!) Even extensive theoreycrafting would not increase his performance by much. He survives the fight barely. His enemy in the video is a level 7 normal undead he needed to kill for the quest.
Then he encounters Ressan the Needler on the field. Ressan is a level 9 rare mob. Laszarus decides to kill him. The smart new player that he is, he first clears the area. Then he starts his first attempt. As you can see, Laszarus did the mistake to pull with Shadow Word: Pain. This mistake seals his death.
But Laszarus doesn't give up. This time he does everything right. He pulls at max distance, casts Power Word: Shield on himself first. Lets his mana fill up again. Then he first casts a Smite followed by a Shadow Word: Pain. He then continues to keep the Shadow Word:Pain on the target while recasting the Power Word: Shield on himself when it falls off. He also allows his health to drop to about 50% to allow for higher dps and more mana, doesn't panic when his health/mana gets low and chains casts without latency delay. Here is the video.
Just barely, Laszarus prevails. This is a high-quality screenshot of his stats after he killed Ressan the Needler. And once more via google.
What do we learn? WoW for the new player is a thrilling experience. He can win most fights he encounters, but some require him to do everything right. It's exactly the kind of game I used to love. Until about level 10. And unless you use heirlooms.
WoW was not dumbed down for the new players. It wasn't dumbed down at all for new players. Or Blizzard spectacularly failed at dumbing down at level 1-10. Honestly, that sounds less probable than it is ... Of course, WoW was absolutely dumbed down after level 10!
WoW was dumbed down for, and more importantly by, the hardcore. Only the hardcore have an interest in leveling extra fast with their twinks.
My interpretation (remains): WoW is maintained by hardcore raiders. And these raiders have no idea about the casual or new-player experience. They are forced by their management to also maintain the leveling game and normal raids and stuff. But they don't really care. They just want heroic raids and high-end PvP. They half-heartedly dumb down arbitrary content trying to fulfill their obligations.
From a hardcore raider's point of view there is no way to make leveling more interesting. A hardcore raider sees no point in solo-kiting mobs for two minutes. It's trivial. That's why they just make it shorter and more trivial, so that it's less of a pain in the ass.
In case you want to test this yourself: just doing quests, it takes about an hour to make it to level 6 and Ressan the Needler (without reading quest texts, of course ..)
In the following posts I give much more proof and high quality videos to further strengthen my point. You should especially take a look at this post.