In light of all the QQ and butthurt about the Garrosh model and all the "OH WHAT A SURPRISE MORE OLD GOD CORRUPTING SOMEONE"
I feel like a Majority of you have no idea about the story at all.
Yes, We have killed old gods before (Cthun and Yogg-Saron come to mind) Since I feel a majority of you have no idea of the story, Why don't you read this http://www.wowpedia.org/Old_Gods
They ruled over Azeroth before the Panethon came and cleansed it. Giving order to Azeroth. They were forced under the surface of Azeroth. IF the panethon had killed them all. Azeroth would have shattered. It's why Cthun was under miles of sand and Yogg-Saron was put in a prison in the mountains of northrend.
from WoWpedia itself "Though chained or dead, their influence still corrupts mortals, and even immortals, to this day."
They are a strong factor in the WoW-Lore and at least 2 of them (that we know) are still around. N'Zoth (Who hasn't been found yet) and Y'Shaarj (Who cursed pandaria with the Sha)
Lest we all forget about the burning legion and http://www.wowpedia.org/Sargeras Basically the SATAN of WoW, But then I guess you would complain about Sargeras in some way shape or form.