What Wyvern Sting does is sleep the target for 12 seconds and then drops a DOT (damage over time) effect on the target when it wakes up. The DOT does 900 damage for a level 60 Hunter.
It's actually a really nice talent, especially in a group setting. It's the Hunter's contribution to keeping a target out of the fight for a bit in the same way a Rogue saps and a Mage polymorphs. In solo encounters it is helpful also, as it lets the Hunter set up for an aimed shot that will awaken the target and start the 900 damage DOT ticking.