Tanaan Diplomat

Or you can farm/buy a lot of [item=128315]

This is one of the achievements for the upcoming 'Draenor Pathfinder' meta, required to unlock flying in Draenor! Take special note that this achievement only requires revered reputation and not exalted.

To fully complete the Draenor Pathfinder meta-achievement, you will need to complete: Draenor Explorer, Master Treasure Hunter, Loremaster of Draenor (A) or Loremaster of Draenor (H), and either Securing Draenor (A) or Securing Draenor (H). You will also need to attain revered reputation with [faction=1849], [faction=1850] and either [faction=1847] (A) or [faction=1848] (H) depending on your character's faction for Tanaan Diplomat (A) or Tanaan Diplomat (H).

There is also mention of a rylak mount, [item=128706] as an additional reward. The ability to fly will apply to all of your 90+ characters after any one of them possesses all the prerequisites (So, no need to grind reputation on every alt that needs to fly), however it will not appear in 6.2.0 but rather in a later minor patch (referred to as 6.2.x). Note that Draenor Pathfinder is account-wide, it has been confirmed that you won't have to 'earn' all of the achievements on the same character but merely possess them on said character.

Quote from the Dev Watercooler on Flying:

In an upcoming Public Test Realm build, we will be introducing a new meta-achievement called Draenor Pathfinder. You’ll earn this achievement in Patch 6.2 by mastering the outdoor environment of Draenor—exploring Draenor’s zones, collecting 100 treasures in Draenor, completing the Draenor Loremaster and Securing Draenor achievements, and raising the three new Tanaan Jungle reputations to Revered. Initially, this achievement will award a rylak mount: the Soaring Skyterror, one of the native beasts that roam Draenor’s skies. Players will remain ground-bound on Draenor until a small follow-up patch (6.2.x), when all players who have earned Draenor Pathfinder on at least one character will unlock the ability to fly in Draenor on all their level 90+ characters.

A slightly detailed explanation;

For the Reputation part of the whole achievement, you need to earn Revered status with three specific Tanaan Factions:

[faction=1847] -> Alliance (Can only be earned by doing Dailies / Using [item=128315])
[faction=1848] -> Horde (Can only be earned by doing Dailies / Using [item=128315])
[faction=1849] -> Cross-Faction (Can only be earned by doing Dailies / Using [item=128315])
[faction=1850] -> (Can form a group and grind mobs for the required rep, be at least 670ilvl)
** Be sure to read this guide before grinding Saberstalkers rep, as the guide will drastically speed up the process for you **
www.wowhead.com/guides/reputation/wod/the-saberstalkers-guide

To become revered you need to earn 21k reputation with each of the required factions, but you don't need [faction=1848] if you are Alliance, or [faction=1847] if you are Horde.

3000 Rep : Neutral -> Friendly
6000 Rep : Friendly -> Honored
12000 Rep : Honored -> Revered

[item=128315] . You would need 21 of these to be exact. They go for around 4-5k each on the AH, or you can farm them but that takes substantially longer and would be easier to just do the dailies for the rep.

If you choose to do the dailies, prepare yourself for approximately 21days of grinding and PVP. Lots of players are in this zone, and they do not think twice to earn a quick 5 Honor Points by ganking you if you are an easy target ;)

You need to earn the reputation part on one character, this cannot be spread across several characters for progression. However, the rest of the Tanaan Diplomat achievement can be earned by using multiple other characters on the same account. One good example of this would be the Draenor Explorer part of Frostfire Ridge and Shadowmoon Valley, if you have a level 100 of each faction, it is best to use the respective faction to do the exploration of those two zones as you will run into other factions which might delay the process for you.