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Reaching exalted with Vol'jin's Spear rewards two achievements,  Hero of Warspear and  Vol'jin's Spear.  The former is PvP -> Ashran while the latter is Reputation -> Draenor, but both have the exact same criteria -- just reach exalted.  (Rather unusual for Blizzard to add two achievements for factional reputation status, but... *shrug*)

 Vol'jin's Spear awards the title <Name>, Spear of Vol'jin.

That's great and all, but what's the best way to grind rep for this faction?

It has been my experience that the best way to gain rep is to do the events, along with killing the enemy leader when you can manage it.  However, this greatly depends on having a good group.  If the group is bad, there is no point in wasting your time in Ashran and you'd be better off leaving until you find a better group at a later time.

Additionally, if you can loot an enemy player before someone else does (and pray it's set to Personal Loot and not some other loot system), there is a chance you'll loot one of the following:

•  Blood Elf Ear
•  Draenei Tail
•  Dwarf Spine
•  Forsaken Brains
•  Goblin Nose
•  Human Bone Chip
•  Orc Tooth
•  Pandaren Hide
•  Severed Night Elf Head
•  Tauren Hoof
•  Troll Feet
•  Tuft of Gnome Hair
•  Worgen Snout

A PvP racial token can be turned in to Chris Clarkie (A) / Angry Zurge (H) for nicely sum of 2,500 rep (or 3,250 if you have Level 3 Trading Post &  WHEE! ).