The tits, son, the tits...
Aside from that, Sylvanas's true agenda is somewhat a mystery - she might turn against the rest of the horde and the belves, the might commit suicide, she might become the new LK, she might get married and spawn more overlords - WHO KNOWS?!
Garrosh was a racist with a global conquest idea - and nothing else.
He simply had no character depth or anything else, just the racism and the desire for conquest.