Seems to me Blizzard is hoping the DK will solve a lot of the perceived balance issue that Arena has, allowing them to fix problem without openly 'nerfing' anyone.
For example,
- Once you take out the inflation of RPM, mage is one of the less represented class in the Arena. A Frost DK is going to be incredibly tough, have almost unmatched control over his targets and open up insane possibility with Shatter Crits if paired with Frost Mage. A DK allies maybe what Mage have been asked for in 2v2 and might give mage more then 1 possible comp in 3v3.
- The DK has several abilities that just yelL '***** you druids!' - Degeneration and the simple fact that he deals with disease (Which are not something a druid can dispel. It's a well known fact that druids are totally out of control in pvp.
- Rogues are also out of control in pvp, and while none of the current DK ability particulary say 'Death to All rogues!', high dps plate class are the usual counter to the rogue class.