After a very frustrating week of trying to ladder with dragon paladin and bouncing between ranks 1 and 4, I switched to warlock and cruised to legend in just a couple hours, going 23-6, including eight straight wins from rank 2 to legend. I started with more of a zoo-like shell but kept tweaking it as I went along (with help from my buddy Tyzor—and clutch coaching from Ted) and ended up with something more like Spark's "Demonic Void" from last season (http://www.hearthpwn.com/decks/209482-spark-demonic-void-s12-demon-synergy-brm-update).
I haven't even finished tuning the deck, but it's clearly pretty good already. The basic concept is similar to other demon-oriented decks—lots of demons, lots of sticky minions, enough flexibility to out-aggro face hunter and outlast control warrior. The main difference with my list is a strong emphasis on manipulating your own board, with Void Terror being the MVP.
I cannot overstress how strong and fun this guy is—he has obvious (and incredibly powerful) synergy with Power Overwhelming, Voidcaller, Nerubian Egg, and Sylvanas Windrunner, but he's also a great way to shift stats on your board to play around your enemy's removal (e.g. Swipe will kill a 3/3 and two 1/1 imps, but it won't stop a 5/5). He's also an excellent sponge for silence and removal (including BGH), which protects your other minions later. And sometimes he's just a massive 12/11 steamroller and they concede on turn 5.
If people are interested in specific matchups, mulligan tips, card replacements, tech, etc. I'd be happy to update with that stuff. For now I'll just say that there's a ton of interlocking synergy in the deck; almost every card interacts favorably with two or more other cards, so be careful about what you remove or replace. The key exceptions to this are the tech cards, Ironbeak Owl and Loatheb. Owl is a champ and I would never play without him, but I'm not entirely sold on Loatheb in this deck, so tinker with that slot if you like.
Sylvanas and Mal'Ganis are must-haves. I don't have Lord Jaraxxus; I would certainly try him if I had him, but I don't think he's essential (that might just be the sour grapes talking, though). I don't think Emperor Thaurissan fits in this deck and I don't think Big Game Hunter is necessary (you should almost always have board control and are generally able to deal with big threats just by running buffed weenies into them). I would much rather have a third PO than BGH, if such a beautiful world were possible.