We've been discussing the whole 'bring the player, not the class' idea, playfully dubbed by Eliah as BTPNTC, and in raids this boils down to a few core abilities or effects that Blizzard feels are mandatory. It was such a powerful statement that it got Eliah's mind doing all sorts of math about it. That said, the same doesn't quite apply to PvP. In Arenas, particularly, some compositions are simply more viable and synergistic than others. Take the incredible cohesion of Rogue-Mage-Priest, which continues to be a powerful comp even in Level 80 Arena play (as seen in the current ESL tournament).
When asked the question of whether BTPNTC applies to PvP, as well, Ghostcrawler had a succinct answer: "we're not sure." He explains that the immediate concern is to make all specs equally viable in PvP (read: Arenas), with particular focus on the class specs that have historically been underrepresented. I can tell you right off that that they probably overcompensated with Survival Hunters. With such limited numbers -- twos, threes, and fives -- it's quite unreasonable to think that just any combination of classes and specs will work the same way they do in PvE.
Ghostcrawler says that one of the coolest parts of Arena play is "building comps with certain synergies". Ironically, that's probably also one of the most uncool things about it. Certain compositions are naturally synergistic and strong that playing outside of the box or cookie-cutter Arena specs (as far as cookie cutter in PvP goes, anyway) actually gimps some classes and players. Some classes are just so strong that some pro players reroll and never look back (i.e., Serennia rerolling a Death Knight) while other pro players just give up (e.g. Korean team withdrawing because of Death Knights).
Class and spec parity is most assuredly a top priority, but the Arena format will never, ever be as class or spec-friendly as traditional PvE content. Arena play demands greater synergy within teams than a dungeon ever will. Ghostcrawler says, "I'm not sure it would be a good design goal to say almost any 2, 3 or 5 classes can form a good team together." Although he mentions in passing that some teams have no healers, it should be noted that in 3v3 -- the money bracket which pro tournaments focus on -- there are no successful pure DPS teams. This means that some classes will continue to be more in demand than others simply as a consequence of the format.
That said, some effects are somewhat 'mandatory' in PvP the way Replenishment is for PvE. For most of the first four seasons, this was the Mortal Strike effect. It was so critical to PvP that the effect was tagged on to Aimed Shot as one of the ways to improve Hunter desirability in Arenas. A current variant of this is the HoT-removal of Death Knights or the HoT-reducing effect of a Warlock's Shadow Embrace. With healers being key to the success of many Arena teams, particularly on the pro level where quick burst simply no longer happens, there needs to be a way to balance things out.
Blizzard seems to be on the right track about prioritizing spec viability, with the 'make tanking more fun' philosophy that resulted in higher tank DPS also having the happy side effect of tanks becoming more viable than ever in PvP. It will be a slow and understandably unending process, but the developers obviously know what abilities or effects will make specs viable in PvP and Arena teams. Short of handing out Mortal Strike to everybody (something they had considered for Enhancement Shamans at one point), it will be interesting to see what other things they can add to make different specs not just viable in PvP, but desirable in an Arena composition. I mean, Dispersion is cool, but it probably isn't the answer...