The new warchief ended up pretty much as expected and as many people were hoping for. As much as I like Thrall, I think it's best that he's not the Horde leader again. He's grown very powerful over the years, and is basically a living legend in the game now.
So, where does WoW go from here? If rumors end up true (and they're looking good so far), the next expansion will likely be named "The Dark Below", which is possibly a reference to the kraken that takes away Neptulon (and is in league with the Old God N'Zoth). It's probable that the expansion will revolve around the plots of the Old Gods, especially since Garrosh was using some of their power.
Speaking of power, I understand that Garrosh can drop BoA weapons that scale from player levels 90-100. Not only is BoA gear (especially for future levels of play) a new thing for raid bosses to drop, but it also hints that level 100 is on the horizon for players. Either your weapon will still be "good" for two 5-level expansions (including through all the raid content - not likely), or one 10-level expansion (up to the end-game raids - which is much more likely).
This hints greatly at a large expansion coming, possibly large enough to include both Old Gods' schemes as well as the return of the Burning Legion (another vastly rumored expansion theme, with grounds for plausibility in-game in several ways). If this ends up the case, the lore will be great. I can only hope the gameplay becomes challenging again...