I, as well, took a long break. I quit within the first few months of MoP, and started up again about a week before HFC came out.
-Don't waste your time with MoP stuff. You can one man Vaults, do Throne with mythic gear, and if you don't get into a good guild, you can 2-5 man most everything if you want to. (Not sure when it happened: Blizzard gave a huge buff to players inside old raids. For example, I 2 shot Hagara with 2 Chaos Bolts.)
-Green fire eventually
Focus on other things first, and get to this when you have a minute. It's really pretty amazing. I wish I could have done it during current.
-Curse is my go to.
-Affliction is really strong in raids for single target. It's really strong in mythic HFC on farm for 90% of the bosses. For progression, Affliction and Destruction are both key. Destruction is used in dungeons as well. Play both, focus on destruction first, as you'll be using that more. Plus, not sure how much you did play in Mop: I played only a bit in the beginning, and I played affliction then, so when I came back, I had to learn what burner embers were and how they worked.
-I'd love to hear someone elses answer on other warlock communities around. I just hang out here once in a while.