It occurred to me that I QQ a lot about Moonkin viability. And although some measure of QQ might sometimes be warranted, I feel that maybe I've done too much. So I decided to make a simple, short post, that sums what what I truly think about the viability of Balance Druids.
Player vs. environment
In PvE, we are fine. I've been paying close attention to my guild's meters (DPS and various others) in our latest runs, and I find myself always in the top 5 DPS of the group. I do at least 9k DPS on every fight, and that's with currently-bugged Starfall and Languish damage.
Though it is true that Moonkins suffer from movement more than most, our DPS is fine, and we bring great raid buffs. In Cataclysm, we are just fine as well, if not better.
In Heroics, we are downright amazing, as our offheals and CCs come greatly into play.
Player vs. player
In PvP, we are OK. I still think that if you wanted to do 2v2 arena, you'd be better off with a Shadow Priest, Elemental Shaman, Mage, or Warlock. However, we are still capable of doing well. Although all of these classes have better survivability, control, and burst, they will never have our excellent mobility and potent heal-over-time spells (though not very mana-friendly). There are also a few things that I do not find very fair (as explained in this post), but again, we are still decent.
In Cataclysm, we are not doing so well. To sum it up in a few words, Cata's Eclipse is worse in PvP than WotLK's, our healing is gimped, and our control still pales in comparison to other classes. Also, our DoTs, which are a much bigger part of our damage than ever before, have no dispel protection, further reducing our damage (every team has a dispeler now), and cutting us off from important Shooting Stars procs. Wrath, Starfire, and Starsurge cast times are sluggish and nearly impossible to land in movement/interrupt-heavy environments, though we depend on them to reach Total Eclipse.
So let us hope for some Cata PvP buffs!
So overall, we are fine (in 3.3.5). We may be slightly underpowered, but it's not the end of the world. I am generally happy with my Moonkin these days. And if you ever question what I think about our feathery selves, come to this post. Ignore the QQ that may have come before.