Many aspects of the Balance Druid have already been greatly dissected and laid out in various places (official forums as well as blogs), so I needn't further flog it.
This is a very PvP-focused post.
The entirety of each Druid specialisation suffers due to the Druid having discrete sets of abilities that barely overlap from spec-to-spec; each spec is not only designed to use only its tree's spells, but also actively discouraged from using off-tree abilities. Balance Druids; Yes, Balance Druids need weaker heals. Given the new healing model of Cataclysm, the once oft-heard counter-argument that 'healing is a Moonkin's defense' is no longer valid.
With healing so low relative to heal pools (excellent design, imo), the Balance Druid has lost their crutch defensive fallback without any new defensive option to replace it.
What Balance Druids are now doing in PvP is 'abusing' Lunar Shower since the option to turret and Wrath/Starfire just comes with too much risk for very little profit.
While I feel that Lunar Shower could be improved slightly ("This effect is refreshed and amplified when you move or cast Moonfire, ..."; strafe-twitching is a clunky playstyle), I feel that it is the reward of turreting that needs attention.
There is too much risk involved in hard-casting.
The dangers of stopping for a Starfire cast is so great that it's a no-brainer.to instead go for the freedom of movement of Moonfire spam. Something is needed to reduce the risk of casting Wrath and Starfire. Even if this comes in the form of a defensive panic button that allows the Balance Druid to have options once they've commited themselves in siege mode. Or simply have better defensive procs ("When you are silenced or a spell is interrupted, you slip into the Dream and are untargetable for 2 sec.")
Moonkin Frenzy is the perfect talent for a PvP-focused effect in this vein. Not only is the pushback protection obsolete, the 10% damage buff while under attack is utterly pointless. When the Moonkin is being attacked, we immediately have to play defensive. Gaining +10% damage while on the defense doesn't help us survive in any way.
Even a mechanic similar to a Frost Mage's "Early Frost" (Reduces the cast time of your Frostbolt spell by 0.3/0.6 secs. This effect becomes inactive for 15 sec after use.) would go a long way to allowing the use of our core nukes in PvP. Or perhaps even a baseline lowering of the cast time of both Wrath and Starfire, to then have a stacking percentage buff for each consecutive cast.
Inevitably, Eclipse comes into this. With our core binary spells of Wrath-Starfire suffering so much, Eclipse is non-functional in PvP and, even more, It is still too much of a damage swing compared to non-Eclipse.
Without being able to reliably proc Eclipse, a Balance Druid is quickly dry. Others have suggested having a cooldown (~1-2mins) that allows us to instantly proc an Eclipse. I do not see this as the fix and instead believe the ebb and flow mechanic can still be worked with once Starfire and Wrath themselves are castable.
Back to my starting point: each Druid specialisation suffers due to the Druid having discrete sets of abilities that barely overlap from spec-to-spec. The Balance Druid has a small toolkit of spells, which is fine as a design point so long as there is depth to that limited toolkit.
My point? In PvP, our current core offensive Balance spells are rigid and restricted, and very often don't even fulfil the role they're designed to do. We either need more abilities to give us options with our playstyle, or simply add depth and reiterate each spell we currently have.