Level 90 Boost and Proving Grounds
You won't be able to queue for Heroic Dungeons or LFR until you have a certain level of Proving Ground achievements.
In a patch after release, Proving Grounds will be made to do a better job of telling you what you are doing wrong, rather than just being a test.
Players that don't have Proving Ground achievements will still be able to go into normal dungeons.
Questing
The Jade Forest quests had a very clear story, but it also had a lot of side quests that could bog you down.
In Warlords of Draenor, your map will show you where to go to continue the main storyline, along with the locations of bonus objectives.
The bonus objectives no longer have any story text that go with them, just a list of objectives. Now when there is quest text, you will know that it is really worth reading.
Garrisons
After rescuing members of the Frostwolf Clan in the Warlords intro quest experience, Durotan will give you a plot of land. This is where you start building your garrison.
After you complete their quests, some questgivers will be available for you to recruit as a follower for your garrison.
You will be redirected to your garrison frequently throughout the leveling experience, as it is integrated into the zones and leveling experience. Every time you go back, you will see familiar faces from your questlines.
Garrisons offer lots of hard choices that give you nice rewards, such as the buildings you choose for your limited plots. Buildings and followers aren't a permanent choice though, so if you change your mind you can change things out.
As you go out and adventure in the world and irritate the enemies in Draenor, they will attack your garrison. You and your followers will need to fend off these attacks.
Ashran
Ashran will have a big central battle that is pushing towards the other faction's base.
Around the outer edges of Ashran you can find some rares and other things similar to Timeless Isle, with greater rewards to be found on the opposite faction's side of the zone.
Misc
There are a lot of advantages to having a realm community, so there aren't any plans to allow players to join guilds on any realm.
When you log in to Warlords of Draenor, you should be able to play with your friends. Allowing cross realm raiding and the changes to raid locks are one way the team is working towards this goal.
The team is still talking about a good reward system for completing older content with scaling, but there aren't any announcements for now. The Level 90 boost allows you to play with your friends without needing to level through the older dungeons now, making it less urgent.