If you can remember Wrath -- I know, it was ever so long ago -- then you can remember the time when Eclipse wasn't always just a flat damage increase. Back then, Eclipse had two different parts. For a Lunar Eclipse, Starfire's critical strike chance was increased; Solar still increased Wrath's damage, as does the current effect. It took a lot of balancing factors to get the two procs in line with each other -- and in fact, they never actually were on the same level, but there was a lot of interesting synergy with the Lunar proc. In late Wrath, druids were easily able to reach over 100% critical strike chance on Starfire when a Lunar Eclipse was up, which actually wasn't that terrible. It did wonders for Nature's Grace and our tier bonus of the time.
Sadly, much of our previous synergy with crit has been removed with this expansion, with the upside that Eclipse itself is now nothing more than a pure damage-increasing buff. This in and of itself is somewhat problematic. Masteries are highly unique attributes; while a majority of them are nothing more than pure damage modifiers, they generally only modify a specific portion of a spec's damage. Assassination, for example, has a mastery that increases poison damage; affliction increases DoT damage; and frost increases frozen target damage. Eclipse, too, is a limited form of a damage increase, affecting only arcane or nature damage and only when either proc is active.
The difference is one of mechanics. Very few specs have a mastery that directly increases their ability to nuke or deal larger amounts of damage in a single burst. For example, elemental's mastery, the ability for a spell to "double cast" for 75% damage, holds a very high burst potential. Getting hit with two Lava Bursts or Chain Lightnings will seriously hurt someone, yet stacking mastery for elemental isn't entirely effective, as it only increases the chance that this will occur, not the actual damage of the cast.
Eclipse influences all of our damage while it is active. Although a druid may not seem threatening while Eclipse is down, suddenly gaining 50% or more damage (which is entirely possible now with some mastery stacking; in future gear, it will be far easier to reach even higher levels) is a far more significant boost in damage than any other mastery out there.
The effects of stacking mastery
I know I breached this subject in the previous article, but it's a point that really needs to be understood. Another comparison: Destruction's mastery is like Eclipse, a flat damage increase to fire damage, so let's see how mastery stacking effects both of them.
Destruction starts with a base of 10.8% on its mastery, and each point of mastery increases that by 1.35%. Say we're stacking mastery and have 20 of it. This would give the warlock 37.8% more fire damage at all times -- a pretty hefty amount of damage. Now, Eclipse has a base of 16% and is increased by 2% for every point of mastery. A druid with the same level of mastery would have 56% additional damage while under Eclipse. If Eclipse had an uptime of 65% during an encounter, then it would equalize out to 36.4% more damage overall, which is relatively close to what the warlock gets from equal amounts of mastery. (It is worth noting that Eclipse usually has an uptime between 65% and 70%.)
In a general sense, this is balanced; both specs receive relatively the same amount of damage from their respective masteries. However, it isn't the equalized damage of Eclipse that is of concern. How can one consider balancing damage values when they can swing in degrees that large?
Striking a balance
Damage balance itself is pretty easy. You pick a target damage range and then you adjust either resilience reduction, base damage, damage modifiers, or coefficients to reach that goal. Yet where should the goal lie for balance druids? Do we balance around damage assuming Eclipse? Do we balance without factoring Eclipse? Or do we sort of pick a random point in the middle and hope it works out?
Balance druids are not feared PvP targets, but they can achieve shockingly high burst seemingly from nowhere. Going cooldown for cooldown against a balance druid in a Solar Eclipse is not smart for most specs; in equal gear and skill, they will likely lose. This is because we have abnormally high burst in Eclipse, because it has to be balanced against times where we don't have Eclipse.
There are far more ramifications to this issue than purely PvP; PvE is impacted by this, as well. Timing is excessively critical, and burst is highly important in a wide variety of situations. We see it all the now in virtually all content. Moonkin will force and game Eclipse so that they can maintain it for specific burst periods, which usually involve AoE.
Some players might say that burst doesn't matter in PvE as much, but that simply isn't true. Heroic V&T shows this. When entering the Twilight Realm, you have two options. You can have a group of sub rogues stay in virtually the entire encounter, as they are capable of being completely self-sufficient for the most part. Without this option, you have to cycle players in and out. If you are cycling players, those players can only remain in the realm for a short period of time, during which they have to kill as many adds as possible before they are forced to leave.
Burst matters here. Yes, the burst occurs over a period of 20 or so seconds, but that isn't nearly long enough to complete a full Eclipse rotation -- barely long enough to do half. The ridiculous damage difference between being in Eclipse and not being in Eclipse for this phase isn't a matter of taking a few extra seconds to down an add; it's the difference between getting two or three additional adds down. It's the difference between a wipe and a kill.
Eclipse needs a solution
The fact remains that the basic premise of Eclipse must change for a variety of reason. Altering spell schools is a good idea on Blizzard's part. The switch from only working with Starfire and Wrath to working with all of our spell schools was a good idea; even turning away from increasing our critical strike chance was smart design. Despite all of that, the alternative that we were left with is no better than anything else.
In this concept, I really have to applaud Blizzard's ambition and ingenuity; the excessively subtle balance of Eclipse is masterful work. The issue is purely one of the right hand not knowing what the left is doing. Eclipse is solely a concept designed for PvE, for which no thought was given toward PvP at all. Not only has this resulted in a flawed PvP system for balance druids, one in which we feel that the entire thing works against us, but it also results in the conflicting issue of Eclipse being far too powerful in such a setting where burst is paramount.
It is not damage that holds back balance druids in PvP. It isn't Eclipse. It isn't even our rather weak control methods. Solve the survivability problem of balance druids, and I can promise you without a doubt they will quickly replace restoration and feral as the druid PvP spec.
Eclipse must be curbed; it must be corrected. We are walking a dangerously thin line by leaving the current system in place. To retain the balance that we have, to keep mastery as an appealing stat and yet totally neutralize its ability to create radical burst situations is a tall order, but it must be done.