Even though most of the details about how to gear up are neatly put together here, I wanted to write a post since a couple of people have asked me about the game lately.
First, Mists was bad for alts. It was extremely difficult to play a alt during the start of this expansion but they have gradually made things easier for alts. Also while the story was very engaging for your first character the quests really did not bring anything new the second time around so there was tons of burnout. So if you leveled during the start and kind of got tired of dailies, it is the perfect time to catch up.
The story has moved forward and in a week or two you can catch up not just gear wise but story wise as well. Domination front and then the Thunder isle will not just take the story further but it will gear you up quite a bit. Also the barrens are a good catch up but there is only a week left before patch. Also a note is that the battlefield barrens is going away forever after the patch drops, so if you want the title and the pet this is the last chance to get it.
If you are thinking of coming back to the game I would suggest that you get your 90 and start domination front and then the thunder isle. You will quickly catch up with the story and be able to gear up. I would also suggest you hoard all the Justice Points and Valor Points until after the patch in a couple of weeks. Once you have gone through that content you can jump into LFR and finish up your gearing. I might be running some older raids next week prior to patch so people that just came back and have not seen it in anything other than LFR can get the experience.
A couple of key things to keep in mind is the introduction of coins in the game. There are now coins that you collect to turn into tokens that give you extra rolls in the current raid. Also the old coins give you extra rolls on the old raids. It can feel like another gate, but for all content it is truly a quick way to catch up. There is also a new loot scenario that will fill up your coins quickly so you can do the old LFR and get tons of loot pretty quickly(see this guide for more details on that). You get one key to that loot scenario minimum every week, if you are lucky you can sometimes get more.
So TLDR get ready to go kill Garrosh!
Garrosh Must DIE