First of all, the Moonkin has high dps potential (related to fact that it is considered to be a hybrid), cause of NG, wich reduces the casttime, without reducing the +spelldmg-quotient related to the casttime. Thats the point, why Spellcrit for a Moonkin ends up with higher value. The dmg of the current spell is doubled (if i`m not wrong its about 210%) and the dmg of the following spell is +33% (u´ll use NG-Procs always on Wrath, to reduce its casttime from 1,5s to 1,0s). So the "real" dmg increase for crit is about 233%. (The "real" math has to be calculated a little bit different, cause u have to consider +hit, +spelldmg and +crit in combination and interdependancies)
That said, one has to consider the big problems of a moonkin.
The biggest Problem for Moonkin is the Manamanagement (Oomkin). With a little bit of crit gear, u can go up to 200 mana/s. U can do the math, how long u can sustain this manaconsumption in a bossfight. With pre-raid-gear, there is no way to compensate this. U have to get ur hands on a lot of epics (mages/warlocks wont be happy), enchantments, u have to farm demonic runes, dark runes, major mana potions and use them heavily, just to get urself through half a bossfight.
The second big problem is Spellhit. Other Spellcasters have talents to get spellhit, the moonkin has not. With ZG not out yet, its currently nearly impossible to stack up enough Spellhit to make decent use out of +spelldmg and +crit as a moonkin. If i`m not mistaken, the absolute number of spellhit needed for a bossfight is 17%. U dont need all 17%, cause if u do the math, every % of spellhit is not worth the previous one, dps-wise, but it is worth getting a lot of spellhit. (Overall Spellhit maintains to be a problem even in late late game, cause as a moonkin u`ll always need a lot more of it than other spellcasters, but this will decrease the possible +spelldmg and +crit on ur gear.)
That said, the conclusion has to be, Moonkin is not the worst spec to get and if u really have the ambition to be a good moonkin, it can get challenging, cause u have to do most of the math, rotations, gear-checks for urself, to get the last % of dps out of ur Moonkin. There r, except maybe some guild forum guides, nearly no guides out there to help u with that.
But it efforts hell a lot of farming, raiding and thoughts to be even a mediocre dps in raids and therefor usefull for ur guild. And without a guild carrying u through instances, u have to start as a healer, cause the pre-raid-gear for a Moonkin only works for half an MC-Bossfight, manawise. (at least for one half of the MC-Bosses). And with ZG not out yet, u will have a big problem when it comes to Spellhit.
Greetings,
Me
(Sidenote: Remember, every other Spellcaster will love ur Aura, when it comes to dps, but u will have to fight with them over the gear.)