So I haven't played wow since shortly after hitting 90, and I wasn't around for any big patches in MoP. However I am pretty invested in the story - I've read all of the books, and will get the upcoming Vol'jin one, but I'm just not sure whether or not that book will be able to bring me all the way up to speed. Would I need to reactivate and do some quest chainz to stay caught up, or has not much relevant stuff been put into the game yet?
Well, in 5.1, we find out what's the Alliance and Horde are doing and follow a series of events that lead into very...interesting actions taking place. One such action sets up 5.2. Check out wowpedia for everything because I don't want to spoil all of it.