EDIT: That's Eleven Class Challenge, not Twelve Class. I must be more excited about Demon Hunters than I thought!
While storming Grommashar, my Blood Elf Priest dinged 100, giving me one level 100 character of every class. I'm heavily weighted in favor of Alliance: Human Paladin, Draenei Death Knight, Draenei Mage, Worgen Warrior, Gnome Warlock, Dwarf Hunter, Pandaren Monk (Tushui, of course,) and Night Elf Druid. Meanwhile, I have my Undead Rogue, Tauren Shaman, and now Blood Elf Priest at 100 (in order of getting them to the level cap, divided by faction. The Rogue was between the Death Knight and Mage and the Shaman was, I believe, between the Mage and the Warrior - if you care about that at all.)
So no Orc, Goblin, or Troll at the level cap (actually, I still haven't gotten either of my Orcs to 90.)
I do still think that the questing experience in Warlords going from 90 to 100 is better than any expansion that has come before it. It isn't flawless - Gorgrond feels kind of half-developed (part of the problem being that you only do half the zone, which half being determined by which building you pick.) Gorgrond's definitely the weak link here, as there's not really much story to it. Actually, I think the "radically different quest experience depending on building choice" could have been great, but there just wasn't enough there to make those individual quest chains all that fun (and it seems like there's not enough XP in them to actually get you through the zone without lots of rest experience.)
But outside of Gorgrond, things are very good. I think Talador suffers a bit from being linear - the Shattrath and Auchindoun quest chains take you along a very narrow path, and there's not a lot of choice about the order in which you do them.
By contrast, I'd say that the top zones are Shadowmoon Valley and Spires of Arak. I'll admit that part of the reason I rank Shadowmoon so high is that I love the Draenei and also the zone is probably the most beautiful zone they've ever made. But it also feels like a really epic story, and the sort of optional quest chain at Elodor, where you discover Socrethar, is a great little murder mystery.
Spires of Arak has a bunch going on, and there's a refreshing amount of variety to it (I actually wish Blizzard would be a little less thematically consistent. I'd love more expansions with totally unrelated threats, like Yogg-Saron and Malygos during Wrath.) You've got the Adherents, the Sethekk, what could be a necromancer from the Scourge, fungal mind control insanity. The only element I wish they'd explored a bit more was the Shattered Hand, which had a pretty minimal presence. That said, it was nice to get a break from the Iron Horde, so maybe that's ok.
I definitely think the bonus objectives and vignettes were great additions to the leveling experience, and I expect we'll see more of those in the Broken Isles. Solo scenarios, likewise, allowed for some very epic climaxes to big quest chains, and they hit a good balance on cutscenes (really the only place they went totally overboard on cutscenes was Uldum.)
I think the challenge in the future is to see if you can make world content good at the level cap. I think the gold standard here is the 5.1 Shieldwall/Dominance Offensive dailies. But I also think that they need to be better at leaving plot threads to pick up in the quests but also being sure to pay those off. Admiral Taylor's possibly be-Scourged garrison is clearly a long-term set-up for something post-Legion, but I think that there were a few elements (like Cho'gall and the Pale) that didn't seem to pay off that much in the end (I know Cho'gall was the mythic-only boss of Highmaul, but big plot stuff should not be only for the elite of the elite.)
I'm also very curious to see how the class-specific content in Legion is going to work out. As great as these zones were, the twelfth time through them, I wasn't exactly engrossed in the quest text. And that's probably not really Blizzard's fault - I'm the one insane enough to get every class to the level cap - but I think class quests could be a great way to make leveling up an alt feel exciting and new. I hope that it isn't just like "get your artifact" and then you get siphoned into the exact same experience for the rest of the expansion.
Anyway, I tried Clarity of Power, but then switched over to Auspicious Spirits. While I'm generally more of a cleave/aoe guy than a multi-dotter, Clarity of Power made Shadow feel kind of thin. Also, I love those spirits flying toward my enemies.