Hearthstone has a bunch of new cards coming, and they are all themed around the Old Gods of WOW and the corruptions that those old gods have visited upon the unsuspecting denizens of Azeroth. So far all the cards we have seen are marginal at best; that is, barring some new synergy they are not serious cards. Fun deck filler only. That is, with one possible exception: C'thun itself.
That doesn't sound good enough. In order to make sure you actually get to deliver that insane finisher you would need to be playing ~10 C'thun boosters in your deck. 1/3 of your deck being weak cards is brutal, and is going to lead to you dying before you get a chance to C'thun for the win.
A very comparable card is Anyfin Can Happen, which requires the paladin player running it to play out 5 bad murlocs in order to power it, but which also does 22 damage immediately and leaves a monster board afterwards. Pretty similar to a 20/20 C'thun, actually. Thing is, Anyfin Can Happen isn't a legendary so you can put two in a deck. That means both that you are *way* more likely to be able to use one early in the game, and that against a control deck you can likely deliver back to back turns with Anyfin and be unstoppable.
If we look at how bad those murlocs are compared to the cultists we have seen it looks like the murlocs are worse on their own. Not much though as they actually can combo with one another before the big turn, so I feel comfortable equating the murloc penalty with the cultist penalty. Anyfin decks need 5 bad murlocs and C'thun decks will need probably 10 bad cultists. So far Anyfin decks have way better staying power and a greater likelihood of locking out the game early. (Early here is turn 10, of course, rather than turn 16).
Now C'thun does have some things going for it. If you can bounce it with a panda or a Shadowstep and play it again, your opponent is dead. If you can double its effect with Brann Bronzebeard, your opponent is dead. If you can land a Duplicate on it, your opponent is dead. There are a lot of ways you can potentially demolish your opponent with C'thun that aren't available to Anyfin, and there are also a ton of cards that interact with C'thun that we haven't seen yet, some of which may make the C'thun deck much more effective.
The trouble with those extra things is that they are finicky. Shadowstep is good and all but unless you want to leave your C'thun out there and risk him getting polymorphed, you have to give up the 20/20 body and bounce it back right away. Rogue also isn't good at control particularly, so it would be a rough build. Brann Bronzebeard is great and all, but how do you keep him alive on turn 9 when your opponent 100% knows C'thun is coming? Panda bounces also rely on either extra mana from Innervate or Emperor, or C'thun not getting blown up when it finally arrives.
So my reaction to C'thun is this: Unless the other cards in the new set do more than just buff its stats, C'thun is going to be bad. Just worse than a Anyfin Can Happen deck, no question. Anyfin Can Happen is good, tournament quality but not broken, so if all the new cards do is give more +1/+1, then C'thun will be a fun casual deck. However, if a bunch of the cards in the set buff C'thun by reducing mana cost, protecting it from removal, searching it out of the deck, or other similar sorts of utility, C'thun will be competitive. There have to be ways for it to be played multiple times somehow, and to do that reliably. That is the key to C'thun being a threat.
I can't imagine that the devs have ignored this line of thinking. C'thun is the marquee card of the set, so they definitely want it to be playable. Assuming then that they plan to publish support for the C'thun deck, and that they want it to be interesting, my prediction is we see a variety of cards that make C'thun better in the ways I describe, as well as a ton of ways to give it raw numbers bonuses.
I will even go out on a limb and suggest that the best cards for doing so will be warlock cards. I think Molten and Mountain Giants are going to get nerfed to smash handlock and keep it from being a permanent contender and the new default warlock build will be a C'thun build. This is thematically appropriate and makes sure the warlock has something good to do. There clearly will be C'thun support across multiple classes, but the smart money is on warlock C'thun being the new big thing. Write that down, and we can see later if my prediction hits the mark.