Our messed up Outland will still exist. But thanks to the efforts of the Bronze dragon Koriaz, we're also getting an alternate version of Draenor, messed up by Garrosh, connected to our own timeline through a giant portal.
So while the our actions on this alternate Draenor won't effect our past, the Iron Horde charging through the Portal will definitely effect our present, which they are charging into.
This is the one thing that I can look at and say "I'm not really thrilled with this." I have always disliked time travel as a story device and this is no exception for me. I just rationalize it by thinking that there really isnt any other way they couldve let us explore Draenor and fight these orc warlords without time travel.