Well, after we got a tease at his artwork, we now know what Y'Shaarj does.
Like all Old God cards, Y'Shaarj, Rage Unbound costs 10 mana. For that, you get a huge 10/10 body and a rather powerful effect: At the end of your turn, put a minion from your deck onto the Battlefield.
My guess is that Y'Shaarj will work best in Druid decks that ramp up mana production quickly to get out massive creatures. Given all the free creatures you will get from this Old God (you'll get at least one even if the enemy manages to kill him before the next turn,) you'll want it to be something beefy. But you'll need to survive long enough to get 10 mana and you'll also hope you can actually fish the guy up.
Another card that was previewed with Y'Shaarj is Ancient Harbinger, which is a 6-mana 4/6 minion who, at the start of your turn, puts a 10-cost minion from your deck into your hand. This guy will clearly be useful in any Old God-based deck, as it'll help you fish up your legendary guy.
Still no word on Yogg-Saron's mechanics, but I'm sure we'll get it relatively soon.