Well, we were about due for a new Hearthstone expansion, and perhaps not so surprisingly, given all the hints about shadow magic and tentacles, this one's Old God themed.
Whispers of the Old Gods is going to be just as big as the Grand Tournament before it. One of the major features will be the addition of Legendary Old God cards. These minions will have several cultist cards associated with them who will buff the Old God regardless of whether you actually have it in play.
So as an example, C'thun is a mere baseline 6/6 for 10 mana, and deals damage equal to its attack split between random enemies when it comes into play. That may sound very weak, but there are several other cultists who have abilities like "Battlecry: Give your C'thun +2/+2 wherever it is" or "At the start of each turn, give your C'thun +1/+1 wherever it is."
Essentially, you'll be able to keep playing those cards over the course of the game, and once you actually draw C'thun, he might be a 15/15 or something that deals that 15 damage when he first pops up.
There will of course be other notable mechanics, but that's the one that they're showing off. They have talked about "corrupting" current cards, though I suspect that means we're getting new cards inspired by old ones, though I could be wrong. As an example, Loot Hoarder becomes Polluted Hoarder, which costs 4 mana but is now 4/2 (still has deathrattle: draw a card.) This could be a stealthy way to nerf these cards, though again I don't know the exact mechanics behind the change.
Additionally, in a patch coming somewhat sooner, there will be new features like 9 additional deck slots (finally) and also introduce the new Standard/Wild modes.