Currently if you kill garrosh on heroic difficulty (old normal) more than once, you're able to bonus roll and get more loot. This includes heirlooms and you can do it as much as you want. I'm not sure if this applies to other difficulties or bosses but I can say that a whole bunch of warriors are doing this for the transmog shoulders right now and it's been confirmed to drop by a few.
I should also add that killing garrosh on heroic is ridiculously easy after 6.0. I just one shot with several pug groups no problem at all.