The Defiler

Completing this achievement should grant "The Defiler" title imo.

For at this moment if you just start the rep farm , you will need 420 wins of AB to get exalted/achievement/tabard .Thats without the bonus AB weekend rep .

General facts and other neat information around Arathi Basin and the reputation grind:

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• Achievements for reaching exalted with [faction=510] (Horde) and [faction=509] (Alliance) respectively awards The Defiler and Knight of Arathor. These achievements are steps in The Conqueror (H) / The Justicar (A).
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• Once reaching exalted a quest will be avalible in Arathi Highlands (Hammerfall (H), Refuge Point (A)). The quests are [quest=8122] (H) / [quest=8115] (A) and awards respectively [item=20131] (H) and [item=20132] (A).
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• Most of the random achievements in Arathi Basin are listed here.
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• Getting exalted with respectively Arathi Basin faction WILL take time. According to parazyte's estimation you'll get 75 reputation per game. So prepare for the grind of your life.
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Reputation gains for full victories (reaching 1600/1600 points) during normal days:

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• 10 ticks × 10 reputation = 100 rep.
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• It ticks every 160 resources you reach.
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  (11 rep per tick = 110 rep in total with [spell=78635]).
  

Reputation gains for full victories (reaching 1600/1600 points) during Call to Arms:

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• 11 ticks × 10 reputation = 110 rep.
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• It ticks every 140 resources you reach.
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  (11 rep per tick = 110 rep in total with [spell=78635]).
  

Can I get more reputation when farming AB than this?
If you're in a guild with [spell=78635] you will be getting 10% extra reputation. Meaning 11 rep per tick instead of 10:

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• During normal days: 10 ticks × 11 reputation = 110 rep for a full victory.
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• During Call to Arms: 11 ticks × 11 reputation = 121 rep for a full victory.
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Does it matter whether you win or lose the BG? Regarding reputation gains.
No, the only thing that matters is whether you reach the required resources or not.
E.g. Reaching 160, 320 etc or during CtA reaching 140, 280 etc. Of course you'll get more honor for winning.

How does this work if you have both [spell=78635] + [spell=20599] (Alliance only of course)?
They should be stacking and you should be getting 12 reputation per tick. Not quite sure about the exact number if they are both based on the base amount of rep or if they are based on eachother.

Are there any quests you can get extra reputation from?
Yes there are three quests. These are the same quests that have been in the game since Vanilla though, so you might have done them even though you might not remember it. The quests are avalible from Hammerfall (H) and Refuge Point (A) in Arathi Highlands.
One NPC per faction gives all quests:

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• Horde: [npc=15021]: [quest=8120] (Neutral required), [quest=8121] (Friendly required), [quest=8122] (Exalted required).
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• Alliance: [npc=14983]: [quest=8105] (Neutral required), [quest=8114] (Friendly required), [quest=8115] (Exalted required).
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There are one or two more quests called Standard/Advanced Care Package but they only award 10 reputation.
And no, Blizzard has no plans what so ever to change this reputation system. They like how this is in AB+WSG and would like to keep it as one of the few über grinds in game. The only fact we can rely on is that RNGesus plays a very little role in this reputation grind...

Updates as of 4.2
It appears that reputation gains has been reduced by approximately 30%.
You now get reputation during normal days for full victories:

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• 6 ticks × 10 reputation = 60 rep. (66 with [spell=78635]).
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• It ticks every 260 resources you reach. (Up by 100 resources.)
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And during Call to Arms:

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• 8 ticks × 10 reputation = 80 rep. (88 with [spell=78635]).
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• It ticks every 200 resources you reach. (Up by 60 resources.)
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This is in my opinion outrageous and I'm hoping it gets reverted.