Looking around the internet I came upon this discussion on the Moonkin Repository. None of the sentiments expressed in the thread are new, but I've grown frustrated with some of the conclusions that have been drawn over time. It seems to me that some people have an unrealistic expectation of what set bonuses should be. Please let me use the 2T8 vs 4T9 debate as an example.
A History of 2T8
The first version of 2T8 was a 6% increase to your Eclipse buffs when the eclipse buffs were mutually exclusive. Math that I and a couple of other people did showed that it was extremely inferior to 4T7 and suggested that Blizzard buff it to 15% to make it competitive. When Blizzard buffed it as we suggested it became a good set bonus that increased Moonkin DPS by about 3% give or take. In patch 3.2 Eclipse was buffed so that the two Eclipse buffs were no longer mutually exclusive, and you could proc one while the other was on cooldown. Not only was this a significant buff to Eclipse, but it was also a significant buff to 2T8. This increased the value of 2T8 so that the set bonus provided about a 6% DPS increase. This is much larger then most other Set Bonuses in the history of the game and many theorycrafters including myself expected 2T8 to be nerfed. It was not.
2T8 vs 4T9: A different perspective
I have seen several comments that said something to the affect of "4T9 sucks. Blizzard is screwing us." I completely disagree.
4T9 is not a bad set bonus. My calculations value it at about a 2.5% DPS increase. Looking at prior moonkin set bonuses and the set bonuses of other classes, set bonuses tend to be worth somewhere between 2% and 4%. 4T9 is on the lower end of that scale, but a 2.5% DPS increase is not something to brush off. It's much more significant than most people think.
2T8, on the other hand, is a bad set bonus. Try and understand what I'm saying before you flame me. 2T8 is obviously a very strong set bonus, but it is an outlier. The reason we don't move from 2T8 to 4T9 isn't because 4T9 is weak, but because 2T8 is strong.
The reality of the situation is that Blizzard is not going to increase the value of set bonuses with each tier of content. It would be an unsustainable cycle. If they did that, at some point only the set bonuses would matter. The trick is to try and maintain the value of set bonuses from tier to tier to make for an easy transition, without making the other factors of our DPS meaningless.
Therefore, every time you say "4T9 is weak" or "buff 4T9", what blizzard probably hears is "2T8 is too strong" or "Nerf 2T8."
Myth: 2T8 was made to Make Moonkin Competitive
It seems to be a popular opinion that Blizzard purposely used 2T8 to make Moonkin competitive with other classes in terms of DPS. Personally I think the suggestion is ludicrous. Why would Blizzard balance any class using a temporary set bonus? It doesn't make sense. To maintain the buff they would have to continue giving that class over powered set bonuses. This would in turn create expectations of what a set bonus should be worth, and the other classes would get up set that one classes set bonuses were always significantly better then their own.
If Blizzard feels the need to balance a class they are going to do so with more permanent changes like having Spirit provide Spell Power, or changing Eclipse so that the buffs are not mutually exclusive.
Like everyone, I would love it if 4T9 was better. I would love it if all moonkin bonuses and abilities were better. I would love it if Moonkin were OP for once in their lives, but that is not good game design. No one likes to hear it but 4T9 is probably closer to what a set bonuses should be then 2T8. If you keep asking for a change to be made, don't be surprised when that change happens on the opposite side of the equation.