Had Thrall died either during WotLK or before Cata & his abdication of the Warchief role, the first thing we can say for sure is that Garrosh would not have been Warchief. Most of the Horde leaders didn't want him as Warchief & really, Garrosh himself only took the role reluctantly. Thrall pretty much had to talk him into it during The Shattering. As we hadn't had a non-Orc Warchief to this point & it took Thrall suggesting Vol'jin to get the first non-Orc Warchief, I think we still likely would've had an Orc in charge. Saurfang would've been a great person for that role, but pending when the killing happened he'd either be busy with the Northrend Assault or mourning his son. Eitrigg might well be the next best candidate.
What that also means however is that we don't really have a good fill in for the Aspect of Earth when Deathwing does his assault. Drek'Thar would likely be too old to fulfill the role, plus his body has been weakened enough that he is using a wheelchair. Magatha is too evil to really be trusted in that role. That leaves us with Rehgar, Muln, or Nobundo to fulfill that role. Of those, I'm not sure who would be most qualified.
Pandaria...it's hard to know how that land would be. Until Garrosh took the Heart of Y'Shaarj from the Vale, that land was constantly threatened by the Sha. The main question here is whether Garrosh would've ever fought the new Warchief in a Mak'gora for control of the Horde like he did with Thrall just before the Northrend Assault. If he had, we'd likely have had everything the same as it was. If not, it's possible Pandaria is still stuck in fear to this very day.
We also have to consider how much this would have changed the Legion invasion. Without Garrosh taking the role of Warchief, it's unlikely we go to AU Draenor, which means it's unlikely that Gul'dan ever gets sent through the portal at the end of HFC. Without him leading the assault, it seems potentially likely that Sargeras would have had time to finish his plans on Argus before assaulting Azeroth, and we likely would have had a hard time stopping him.