The Iron Horde
With the help of the Bronze Dragonflight, Garrosh Hellscream has escaped his trial at the hands of the present-day alliance and horde, and returned to Draenor before the events of the original Warcraft: Orcs and Humans. There, he succeeded in his aim to prevent the pact between the orcish clan leaders (the titular Warlords of Draenor) and the demonic Burning Legion. The result is still a bloodthirsty orcish horde prepared to invade Azeroth, but one without infernal demigods pulling the strings.
The other major difference is that this horde from 35 years in the past isn't invading the Azeroth of 35 years in the past. Garrosh's new Dark Portal has been chronologically aligned to present day Azeroth. As in, the one we've been playing in, which just endured the events of Mists of Pandaria. Yes, it's a little bit more complicated than past Warcraft plots. But who cares? Grom Hellscream is back!
Grom Hellscream is back!
So are Blackhand, Orgrim Doomhammer, Durotan (father of Thrall), Ner'Zhul (pre-becoming the original Lich King), Gul'Dan, Kilrogg, Cho'Gal, and the whole gang. Some of them will be openly adversarial. Gul'dan apparently still "drank the Kool Aid," though he failed to get the rest of the Horde to do so.
Oh yeah. That means alternate timeline Archimonde, Kil'jaeden, and Mannoroth are out there, too. Lovely.
A few of the warlords, however, will be more amenable. Durotan, upon meeting his son from the future, will be torn between following his brethren in the Iron Horde and joining with the Regular Old Horde... from the future... Yeah, I told you it was a little bit confusing.
We have been assured there will be no doubles—any character from the present who enters Garrosh's alternate past will not find their past self there. We haven't yet been told why this is the case.
Explore Old Draenor
Draenor will consist of seven zones—some easily identifiable with their shattered equivalents from present-day Outland, and some less so. The devs describe Draenor as a "world of giants" and "more savage" than Azeroth. Civilization is the exception, rather than the rule, and there are no great kingdoms or empires to hold the wilds at bay.
The two familiar zones are Shadowmoon Valley and Nagrand, though each will differ significantly from their post-shattering counterparts. What was to become Zangarmarsh in Outland is actually a vast sea on Draenor, which evidently drained when all of the water fell off of the broken continents. Terrokar Forest will be known as Talador, the seat of the Draenei civilization. What we knew as Hellfire Penninsula, apparently, used to be a vast, tropical rainforest called the Tanaan Jungle. The remaining zones, Gorgrond and Frostfire Ridge in the north, will be explorable before they collided to form the Blade's Edge Mountains.
You will be flung into this world by a new Level 90 starter experience which was likened to the Death Knight beginning zone in Wrath of the Lich King. For the first time in any WoW expansion, each faction will have its own starting zone. The Horde will begin in the lands of the Frostwolf Clan in northern Frostfire Ridge, while the Alliance will begin in Shadowmoon Valley.
New group content
Warlords will feature three leveling dungeons, four max level dungeons, two raids, and a heroic version of the classic Upper Blackrock Spire at launch. The raids will be divided into four new tiers based on Blizzard's new raid group philosophy. Raid Finder, Normal, and Heroic will now all use the Flex Raid system introduced in the recent Mists of Pandaria patch, allowing you to raid with a variable group size up to 25. A new, non-flex tier for the hardest of the hardcore, Mythic, will be added to the top of the list, tuned for exactly 20 players.
Quality of life improvements
Hit and Expertise stats? Gone. They will be replaced on gear by things like Move Speed and Chance to Cleave. Heirlooms, toys, and possibly tabards will be turned into collections, like mounts and pets are now, so they won't take up bag space. Quest items will no longer take up space in your bags.
A new screen called the Adventure Guide is being added, which will recommend what you should be doing to progress your character's level, gear, and professions. This will include a list of raids near your level, rating them by how difficult they would be with your character and item level. It will track PvE and PvP progression separately.
Best of all, it looks like they finally got around to fixing the dragon-related damage in Stormwind.
Keep an Eye of Kilrogg out for more details on the expansion as they emerge.