If you've been living under a rock, then it will be news to you that we will kill Garrosh at some point in MoP. The question is what will cause him to become so powerful, that you need 10/25 people to take him down? There are several things flowing around, and I personally am hoping for the demon one...
Demonically Empowered (Burning Legion): As it is said, "Garrosh has been doing some bad things under Org", well those bad things include lots of demonic influenced things. This is pointing towards becoming demonically empowered similar to his father Grom Hellscream.
Touched by the Sha (Old Gods): As you see all over Pandaria the Sha are sneaking their magic into just about everything. With someone as hot-headed as Garrosh, perhaps it will be his emotions that cause him to become tainted by the Sha.
Blessing of the Thunder King (Mogu/Divine Bell): Not much is known about the Mogu, other than they ruled with an iron fist, and were lead by the "Thunder King". POSSIBLE SPOILERS FOR 5.1 AND BEYOND AHEAD... It is pretty obvious we will confront the Thunder King come 5.2, so maybe he will have an influence over Garrosh's way of leading the Horde?
Combination: Demonically Sha Garrosh... with a bell collar.
Other: Or we will find out the above has NOTHING at all to do with Garrosh, and we are introduced to something new.