Its late at night so I'll leave you with a small list of the good Garrosh has done for the Horde, whether you accept it or not is a different problem altogether:
1. Reinforced Orgrimmar = Better protection for the horde races that call the city home.
2. Advances into Ashenvale have secured vital lumber for the Horde.
3. Brought the Dragonmaw into the Horde.
4. Brought down the staging point of any future Alliance attack on the Horde in Kalimdor (Argue the means all you want, it needed to be done to protect the Tauren, Trolls, Orcs and Goblins from alliance invasion. Remember Taurajo.)
5. Ordered a more mechanised military and viable navy for the Horde.
6. Successfully led the Warsong Offensive to victory against the Scourge (Don't bother arguing whether you think this is correct, lore says he was praised as a huge success for his Northrend campaign)
7. Brought the Houjin Pandaren into the Horde.