Okay. Warlords of Draenor. There's obviously a TON to talk about, but how do you even start? We're in speculation/information overload!!
So why not start with the lore. Most players know who Grom and Durotan are, but how much do you know about Killrogg or Kargath? About the Bonechewer or Thunderlord clans?
If you want to start reading about the characters who might be major players in Warlords of Draenor, I suggest reading the following Warcraft novels! (Which are also listed in chronological reading order.)
(I've listed the characters who might potentially make appearances in the expansion, but left out characters - even if they were important in the novels - who are unlikely to show up in-game, such as Thoras Trollbane or Anduin Lothar.)
Rise of the Horde
All about the original events on Draenor, amazing focus on important orcs like Ner'zhul, Gul'dan, Grom Hellscream, Blackhand, Orgrim Doomhammer, Durotan, Draka, Geyah and Drek'thar. And just as importantly, huge character details about Velen and the Naaru, who I imagine will play major roles in the expac. A MUST-READ if you want to know who's who on Draenor!
(Ironically, it seems the focus of the entire expansion will be reversing/avoiding the events of this novel, so you should know the history that Garrosh is trying to prevent!)
Unbroken (read online!)
Not a novel, but a short story about the draenei (Broken) dealing with what happened in Rise of the Horde. Akama and Nobundo are major characters that I am sure we'll see in WoD, while Grom Hellscream and Velen also play small but important roles. Additionally, any of the other Broken in the story could definitely turn up in the expansion.
The Last Guardian
The story of Medivh, starring Khadgar and Garona. We'll see Khadgar for sure in Warlords, and probably Garona. As for Medivh? Well, read the novel...it's possible that he might be interested in an alternate-reality Draenor where things went differently.
Tides of Darkness
The major orc assault on Azeroth, and the formation of the Alliance. Major players include Doomhammer, Rend & Maim (sons of Blackhand), Gul'dan, Cho'gall, Zuluhed the Whacked, Nekros Skullcrusher, Teron Gorefiend. On the Alliance side, Turalyon, Alleria Windrunner, Khadgar, Kurdran Wildhammer. I don't know how many of them will be in WoD other than Khadgar, and even that doesn't quite make sense, but we'll see who else shows up!
Beyond the Dark Portal
Ner'zhul rallies the Horde and tries to do bad stuff, the Alliance come to Outlands to try to stop them. There are many potential warlords to learn about here: Ner'zhul, Cho'gall, Teron Gorefiend, Grom, Killrogg Deadeye, Kargath Bladefist, Dentarg, Rexxar, Hurkan Skullsplinter, Fenris Wolfbrother. Young, Red Pox-ridden Garrosh Hellscream also makes a small appearance.
Opposing them are Turalyon, Alleria, Khadgar, Kurdran, Danath Trollbane, Nemuraan. And on no one's side, there's also Deathwing, Nefarian, Onyxia, Sabellian, and Gruul. I expect we'll see Gruul in WoD, but I dunno about the Black Dragonflight, as that's getting into odd timeline overlap stuff. But who knows!
Day of the Dragon
The imprisonment of Alexstrasza by the Dragonmaw Clan. Lots of insight into this particular, infamous orc clan, and in particular its leaders, Zuluhed and Nekros. Most of the other characters wouldn't really have any reason to show up in Warlords, but it's possible Vereesa Windrunner (who is a major character in the novel) might, given her current significant status in the Alliance. (There's not any REAL reason for her to go to Draenor specifically, though...)
Lord of the Clans
Thrall's biography, basically. Lots of info about Thrall, if you needed it (does anyone at this point?), but there's also extensive content about Doomhammer, Grom and Drek'thar, who no doubt will all be important in Warlords. There's also a tiny bit of Durotan and Draka.
Of Blood and Honor
Human paladin Tirion Fordring and retired orc warrior Eitrigg examine what honor really is. This story is largely about Tirion, but Eitrigg will undoubtedly be around in Warlords, so it's worth checking out.
War of the Ancients trilogy
It's possible that orc warrior Broxigar from the War of the Ancients trilogy will appear in Warlords. Really though, these books are all about night elves, Azeroth, the Burning Legion, Tyrande/Illidan/Malfurion...basically everything BUT Draenor or orcs. Still, it's possible I guess, and Mannoroth does appear, the demon whose blood the orcs will be prevented from drinking in WoD. I don't think there will be a lot of expansion-relevant info in this series, but if you're really set on reading up on every possible character who might show up in the expansion, you might take a look.