So, we've got spec out of the way. Let's talk a bit more about gear and habits.
Now, I've seen a couple posts on the WoW forums and elsewhere saying that the order in which you need stats is (in order of priority) Spell Hit, Spell Damage, Spell Crit. Usually, Int/Spirit/MP5 usually are tossed in somewhere after that in varying orders.
The problem that I have with most of these is that, in all the math examples I've seen, they do not calculate in the effect of some of the most essential Balance talents.
For example, Nature's grace. Reduces the next casting time by .5 seconds. Let's say that I'm casting Starfires, and I'm getting about 1600 damage per cast, non crit. My crit rating for starfires is, say, 20%. So, ten casts later:
8*1600 + 2*3200 = 19,200 damage /30 seconds = roughly 640dps.
Now, if we calculate in Nature's grace, we save 1 second of casting. 9,200/29 sec = 662 DPS.
Let's apply it to another 30 second scenario. Wrath this time, say 1000 damage, 2k on crit, over 30 seconds. For the sake of simplicity, let's keep the 20% crit rate.
16*1000 + 4*2000 = 24,000 damage / 30 seconds = roughly 800dps. Factor in Nature's grace, and it becomes 24,000 damage / 28 seconds = 857dps.
So basically, the more you crit, the more the time reduction boosts your damage output per second. Thus, spell crit scales not just because of higher damage hits, but because of more frequent hits.
So, what's most important? I don't think you can say one singular thing, as with our talents there are multiple paths to acheive the same goal. There are, however, a couple things to pursue.
1) Spell damage - Any way you can get it. Because of the large number of Balance talents that boost spell damage you have, moonkin scale very well at 500+ spell damage.
2) Intellect - Raw mana pool, spell damage, crit chance, and mana regen all in one. Intellect is your god. Don't neglect other areas, but make sure you've got a bunch here.
3) Spell crit - because of balance talents, spell crit becomes more valuable to us than many other classes. Doubled crit damage bonus + nature's grace is a wonderful thing.
4) Spell hit - with a caveat. If you have 2 points in Balance of Power, your spell hit will cap out on raid bosses at about 150 spell hit. There is never a reason to get more.
5) Mana/5 - Neglect this at your peril. We've come a long way, but mana issues can still plague moonkin specced druids. Do your math to make sure you have the endurance for long boss fights.
6) Spirit - With appropriate talents, this will add to your mana regen during combat. However, it's a far step down from intellect in most cases.
7) Spell penetration - I've seen conflicting information, and haven't been able to figure out a way to test it myself in any reasonable experiement, so let me just say this: There is a good amount of evidence that spell penetration has absolutely no effect in PvE. There is still some lingering doubts, so don't take that as gospel - but I personally will be investing in Spell Hit before penetration.
My personal take? I prioritize Int and Spell damage, put spell crit and hit on about the same level, though as I gain more spell hit I will be focusing more on improving crit.
So, that's what you're looking for out of your gear. So what is a typical fight like?
There are two cast sequences that I use. Since I generally don't have mana issues, I tend to use the slightly less mana efficent, higher DPS sequence.
Open with Faerie Fire, but do not repeat until it is down
Insect Swarm, Moonfire, Wrath x6, repeat
For a still high DPS, but saving a bit on mana efficiency, go with the following
Insect Swarm, Starfire x4, repeat
Theoretically, the starfire sequence DPS will catch up with the wrath DPS. However, this will be around +2000 Spell damage, so don't hold your breath. :)