Tree of Life and moonkin were denied the ability to DPS or heal (respectively) in forms. The root of the current problem is that, while this replicated the general weaknesses of Bear and Cat forms -- which do not have access to the majority of Druid abilities while shapeshifted -- it didn't copy the general strength of the feral forms. A Feral Druid has access to tanking and melee DPS capacities in these forms that cannot be duplicated in caster; you can't tank in caster, and our melee damage is hilariously bad there as well. The same is not true of a Balance or Restoration Druid (although at this point I think it's reasonable to argue that a Balance Druid gains considerably more from being in form than a Resto Druid does).
Another problem necessarily arises from the fact that Tree of Life was never designed with PvP in mind. For most of BC, its limitations seemed tailor-made not to be of any reasonable use in arena given the speed penalty, the inability to decurse/depoison/Barkskin, and its vulnerability to the Warlock's Banish. This gave rise to the legendary 8/11/42 and 11/11/39 specs that decimated 2's in Seasons 3 and 4; it was universally agreed that the Tree was worse than useless in nearly all PvP contexts, with one of the most frequently-cited reasons being its lack of ready access to the offensive and crowd-control spells that could often decide matches. While the Tree's other PvP weaknesses have been addressed, this remains a problem; other arena healers are not similarly encumbered.
TO TREE OR NOT TO TREE; THAT IS THE QUESTION
If healing-per-second is the only consideration, a Restoration Druid who's not in Tree form can still pump out a chart-topping (or at least competitive) amount of healing, sacrificing only the +healing bonuses from three talents (Master Shapeshifter, Tree of Life, Improved Tree of Life) while preserving mana efficiency. With the -20% mana discount to HoT's now associated with the talent itself rather than Tree form, Tree has lost its primary draw. That's good in the sense that the Druid is not unduly hobbled if they have to drop form -- and that was, I believe, the original intent of the change (an elegant solution for the constant shifting Restos orginally expected do for arena) -- but bad in that we have a form that's now in search of a meaning to exist.
For PvP purposes, the +armor bonus from Improved Tree of Life is too valuable to sacrifice as of the time I'm writing this (Season 6) -- but it means giving up all manner of offensive capability and all crowd-control outside of Nature's Grasp, a choice no other healer is forced to make. This leaves us with the specter of a shapeshift form that has minimal impact on PvE performance (with no new abilities of its own and very few talents affected by its use) and an undesirable impact on PvP performance (where use of the form is virtually required for survival, but forces you to shift all responsibility for CC and DPS to your partner or team).
I could be wrong, but I think that's where Ghostcrawler and the developer team are coming from; Tree of Life has evolved into something that's an awkward fit for what it was really intended to do. The healing bonuses granted by the current incarnation are nice but not game-breaking (which is something I think we can live with, as the talent is otherwise solid), but Blizzard is pretty clearly balancing arena Restos around the use of a form that, for most of its existence, was an incredibly bad fit for PvP gameplay. Yoking the mana discount back to he use of Tree form would make the Tree more valuable for PvE play again, but would exacerbate present PvP concerns about being "locked" in Tree. Giving the Tree access to all Druidic caster abilities is another solution that would help PvE gameplay for those inevitable Oh S**t moments, but might well result in an armored, invincible arena healer with one finger on the Cyclone button and the other on Entangling Roots. It might be appropriate to elect this solution while eighty-sixing the +armor bonus from Improved Tree of Life, but I don't even pretend to know what result that might wind up having on arena balance.
I would advise all interested Restos to read the linked forum thread and/or drop a comment there or here; while Blizzard isn't committed to changing how the Tree works, I think it merits discussion.
*At least til Wrath hit, where the animation was used for several flying mobs and noncombat pets.