Eclipse is probably a rather worn topic for many balance druids. People love it, people hate it. The new design is great, the new design is terrible. It's too mathy, it doesn't function well in PvP, it's too gamey. There are a lot of positive and negative things that can be said about Eclipse. No matter your side of the fence, there are always attempts to improve upon Eclipse, to make it more viable for PvP or less annoying in PvE.
A fair warning, as I attempt to deconstruct both the Eclipse system itself and the common suggestions that arise: It will be impossible to avoid all math. Theorycrafting may not be everyone's cup of tea, but I promise to keep it as minimal and easy-to-understand as I possibly can -- for both our sakes.
Haste and Eclipse
Before we begin, there are a few things that need to be understood about Eclipse, the first of which is haste. Haste is an amazing stat for balance druid. It does all sorts of fantastical things and is without a doubt our single best secondary stat. For all that haste does, the one thing that haste does not do is have any impact on Eclipse whatsoever. Increasing your haste does not increase your Eclipse up time in any way, shape, or form. The factor of increased Eclipse DoT uptime due to the shorter time spans is an inconstant variable that can be altered by encounter mechanics just as much as it can haste.
Eclipse is a factor of spellcasts, not a factor of time. The number of spellcasts required to flow from one Eclipse proc to the next can never be altered outside of Euphoria, and therefore, the time between each proc is a rather meaningless number. While not entirely Eclipse-related, there is an upside to this. Unlike other casters, haste also has no negative impact on our mana consumption. Usually, the faster that spells are cast, the faster mana is consumed; however, our mana regeneration is also tied to the number of spells cast, and therefore, time is also meaningless in this respect.
One of the most frequent ways of modifying Eclipse is via the function of the mastery stat itself. Right now, each point of mastery provides an additional 2% damage toward the Eclipse buff. For a large number of reasons, this is considered as much of a hindrance as it is a benefit.
The primary reason that having mastery increase Eclipse's damage contribution so directly is that it creates the same flawed system that balance druids saw throughout Wrath: We are incredibly powerful inside of Eclipse and incredibly weak outside of Eclipse. In particular, this has very pressing PvP concerns.
For the most part, balance druids are considered to be much of a threat in PvP. Be that as it may, when I'm on my rogue or warlock (who generally chews through moonkin like a toy), I think twice before tangling with a wayward space chicken when I see that golden sun on their bars. Even in a 1v1 situation, a balance druid already in Solar Eclipse is a terrible thing to mess with because the vast majority of their abilities are going to pack a punch.
Sunfire, Insect Swarm, and Wild Mushroom can be rather destructive, and even after the nerf coming with the next patch, any Shooting Stars procs are going to hurt. Factor in that they might have Starfall and Force of Nature up as well, and there's good reason for anyone to give pause when attempting to go toe-to-toe with an Eclipsed druid.
Although the burst of Starsurge is being reduced in the coming patch, this change actually does very little to address the actual issue at hand. Does Starsurge hit too hard? Possibly, but that's an entirely different philosophical matter. What is important to understand is that Starsurge was never the true source of the burst damage, and the meager reduction won't solve anything.
Mastery is the real issue for PvP balance damage. As mastery continues to grow, as druids are more able to stack it, being in an Eclipsed stat becomes more and more destructive. Yes, a Solar Eclipse rotation is only ~80% of the DPS of a "standard" rotation -- but how many other specs can say that they can deal 80% of damage of their normal rotation using nothing but instant spells?
Further, using this tactic would actually result in higher total damage if DoTs can be spread to at least three targets. Factor in high mastery stacking, and you'll face a significant amount of damage.
Think on this: By gaining five mastery, a balance druid's Starsurge will hit just as hard as it does now in the new patch. Is that really such a huge deal? Yes, five mastery is a decent chunk of itemization, but consider that it would also result in all of their other Solar-based abilities to hit harder as well.
As mastery continues to increase, balance is only going to see the issue surrounding Eclipse increase as well. The gap between Eclipsed damage and non-Eclipsed damage will keep getting larger. When you reach a point that 50% of your abilities are hitting 60% or even 70% harder than normal, how do you balance against that?
This is the flaw of Eclipse -- there is simply no balancing rhyme or reason to it. In a PvE setting where you can mathematically balance around uptimes, Eclipse can work. In PvP where every single balancing rule is turned on its head, Eclipse just doesn't function -- not because it is difficult to proc, not because cast time spells are too hard to get off, but simply due to the fact that it is such a tremendous amount of damage. Think about it: Starsurge is being nerfed by only 10% because it deals too high a level of burst damage. Now, 10% is a very big deal; 10% is the difference between too much burst and just enough burst. How then can you possibly consider a damage swing of 50-60% or more remotely balanced? Either we'll end up far too strong in Eclipse or far too weak without it.