@Oracle: You don't see the C'thun decks as a positive thing? The 2 mana 2/3 add +2/+2 to C'thun is too slow for most decks, the 3/5 Taunt is definitely too slow, the 3 mana 2/1 deal +2 obviously is an okay card but easily disposed of. The 4 mana 4/2 divine shield however is a fantastic card, that I'll agree with. And yeah Dark Arrokoa is fantastic.
C'thun decks are a fantastic way to teach new players about slower play, tempo, and curving out decks. It's a F2P alternative for sure. The less you want to spend on this game, usually you have to use very boring fast decks like Face Hunter. Now - face isn't always the place.
Except C'thun Druid has essentially become the new Secret Paladin of the perfect curve deck.
Turn 2: 2/3, +2/+2 to C'thun
Turn 3: 2/1, 2 damage, +2/+2 to C'thun
Turn 4: Klaxi Weaver, 4/10 body
Turn 5: Face Cat, 4/4 charge, OR Taunt Bear, 4/6 taunt
Turn 6: Dark Arakkoa, +3/+3 to C'thun, 5/7 taunt
Turn 7: Twin Emperor, 2 4/6 taunts.
Turn 8: Why the fuck haven't you conceded yet?
C'thun Priest can stall the living shit out of any game, combined with all of the high defensive stats playing right into their hands for easy board trades and maintaining tempo.
C'thun Warrior is just obnoxious because of aforementioned stalling out the kazoo, except with an even better backup plan of fatigue to victory.
I have no problem with slower decks or slower metas, I think N'Zoth Paladin is a perfectly acceptable slow deck that's also competitively viable. The problem is that C'thun decks play like midrange or tempo, but have a control/combo end game. It essentially took the weakness of midrange and tempo, control matchups that survive their pressure and just exhaust their resources, and threw it out the window. And since midrange and tempo have better tools for dealing with early aggression compared to control decks, essentially C'thun decks have the safest overall matchups.
Most professional players agree - if a card doesn't have good enough stats to be played without the buffing of C'thun, it's not to be put into a C'thun deck.
Beckoner of Evil is too slow to play in most decks, or there are at least better choices.
Disciple of C'thun works well only with decks that prefer dealing damage immediately rather than getting a body on the board. Example being rogue. Twilight Elder is just a much better body for the mana cost.
Klaxxi - well I mean, we're no strangers to 4 mana minions with 4 attack and huge health pools. The condition that needs to be met to play it is however more conditional than dropping a twilight drake (Though in fairness, twilight drake is much weaker to silence than Klaxxi)
Dark Arakkoa - Great card, no argument there.
Twin Emperor - Better card than C'thun himself, no argument there.
C'thun - yeah.
That being said Druids did need quite a bit of love to stay relevant since their number #1 win condition was nerfed to the ground (FoN + SR), and on top of that they nerfed Ancient of Lore AND Keeper of the Grove to the point they aren't playable.
Warriors lost Death's Bite and Shieldbearer, and they'd rather have Death's Bite than this new 6/6 give yourself 10 armor if C'thun has been buffed. Control Warrior has too many staple cards to fit in C'thun buff cards. Which means if you want to run C'thun Warrior it's forced to be a fatigue deck, which tend to be very weak against combo decks.
Priest they have a million ways to gain health back but at least they can't overheal like Warrior. With as many big finishers like N'Zoth Paladin (as you mentioned), they can go from 30 to 0 very quick, and it's not like they have light bomb anymore.
I don't think C'thun decks are the safest by any stretch, I just think they are, like you said, easy to pilot. Again, much like Secret Paladin. There's something to be said about how much easier it is to play around a deck archetype when you know exactly what's coming and on what turn. And Shaman is just so damn fast now that people are playing that class now more than they ever had, which just so happens to punish greedy cards like BoE.
A great well thought out post but I don't want to make this post huge so I'm cutting it.