I do honestly believe that Fire was the only build intended to best a Unholy DK in a true PVP situation. Fact is, Unholy is a beast versus all casters. Rogues and Warriors bring them to their knees though, and a Paladin can last for days against them.
Fire does have some nice moves against DKs, but in the end the survivability just isn't there. Lets face it - until you're 80 and you start getting Stamina by the boatload and you can actually get some PVP gear, you're basically a 3 shot kill for a DK - so even with the best of abilities if you fowl up once you're done. They are actually addressing fire for the Next patch - so there you go. I fully believe that they will Balance fire to be able to take on Ret Paladins and Unholy DKs.
This is still a game of Rock-Paper-Scissors... but now instead of it just being class based its much more Spec based. As PVE Arcane I'll take Marks/Surv to the ground, BM still rocks my Socks. With a much bigger HP buffer and a huge buff to Mage Armor, Affliction is less of a concern, where the shear damage output of Destro+Demo can blow a hole in my Face. Ele is a Joke, where Hance will wreck me. I can kite Fury for days, but Protection lasts too long and Reflect ruins my day and Arms can straight up dominate me. Downing blood or frost is a walk in the park, but Unholy just doesn't give me a fair chance. Rogues/Ferals make Mage stew out of me, and Boomkins I find myself on equal ground, but I'm one of the few that can put out the burst and interrupts needed to take out a Resto Druid.
If you look at these same class/specs from a different Mage spec, the whole thing shifts. Frost often has the exact opposite experience. Of course a TON of this is up to skill - but knowing your enemies and who has a real upper hand over you is probably the best thing you can learn, whether its for WPVP, Battlegrounds, or Arenas.