Currently there are PvP power gems, but they are only at about the same power as the primary stat for most classes so no one is using them. However, in MoP these gems double in value, and we gain PvP Power on weapons. Word is being spread that going full PvP power is the way to go, seeing as much as a 50% increase vs. players.
This is where a problem that no one else seems to be noticing creeps up. A flat damage increase is much different that a Str, Agi, or Int increase. Two things happen here; base damage will scale, and stat increases gain power.
1) Base damage.
Melee is out of luck on this one for the most part, as they are based on weapon damage which sees a massive drop with the PvP version. Casters however will see the base damage of their spells scale up with PvP power. It isn't much, but many 2-5% increases have completely changed the game before. To demonstrate with terrible numbers:
A melee attack hits for 10k in PvE gear. All scaling goes down in proportion with PvP gear. It hits for 8k in PvP gear but gets 25% increase from PvP power so they still hit for 10k
A caster hits for 10k in PvE gear. 2k of it is base damage, and the 8k is lowered by the same 20% as the melee's damage from using PvP gear. The total damage is now 8400. BUT they get the same 25% damage as well, resulting in a total of 10500 damage.
Again, these are nowhere near actual numbers, but the idea will remain the same. Classes with high base damage will see a higher return from PvP power.
This one is easier to demonstrate and understand. Lets say A hits for 10k with 10k Attack Power. Player B also hits for 10k with 8k AP, because of his PvP gear having lower stats but pvp power giving him 25% more damage.
They both have a trinket that grants 2k AP. Player A now gets a 20% AP increase and hits 20% harder. BUT player B now gets a 25% increase and hits 25% harder.
In actual gameplay the numbers will be smaller, but can quickly become very significant for certain classes. It all depends on who has the highest base damage on abilities, and can get the most procs. I haven't figured out who will benefit the most from this, but it's obviously a human caster, and likely a Mage spec.
A flat % increase makes abilities with high base damage scale better. Casters get even more OP and procs are more powerful. Classes that can combine the two may see as much as a 10% damage increase.