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except that you can't cast Nature's Grasp while transformed
No.
You can cast it. As far as I know the only forms in which you cannot cast it are Travel and Aquatic. Probably Flight too.
As for the OP, you may find Nature's Grasp to be very useful, cause it really is, but just keep in mind that every point you spend outside of the Feral tree slows down your progression toward Mangle. Of course, if you plan to respec Balance around 40-50, it doesn't matter since you won't be getting Mangle before respeccing anyway.
Umm, I dunno how I should comment your builds since you have a full Feral build up to 70 even though you say you'll respec at 40-50... but anyway.
First build:
-Feral Instinct is usually better than Thick Hide for leveling. Thick Hide is only really useful for end-game tanks. It's not bad otherwise, just maybe not as good as Feral Instinct which will let your sneak on mobs more easily without being detected.
-You might want to put 1-2 points into Improved NG so that it procs faster. 35% base is good but not great, especially not against foes with big, slow two-handed weapons.
-The rest is pretty much a cookie-cutter Feral build. Just remember that Thick Hide and Survival of the Fittest are mostly tanking talents and Primal Tenacity is mostly a PvP talent. If you don't plan to do those much, you can safely drop them in favor of talents that would benefit you more.
Second build:
-Imp NG doesn't need to be maxed. I'd put 1 point in it to put NG at 50% proc rate, and maybe 2 if I had an extra point left from somewhere, but no more is needed.
-Starlight Wrath: You need it. It improves the casting time of your main nukes so this will quickly become your friend as you get to cast more spells in the same time frame.
-Celestial Focus: Might as well max it out. Take a point out of Imp. Nature's Grasp if needed.
-Moonglow is nice if you can afford it, but far from vital.
-Drop Furor. Since you have no point at all in Feral you might as well get Imp MotW for a few bonuses. The only thing Furor could do for you is allow you to shift out of Moonkin, shift into Bear, Bash and shift out to heal.
-Natural Shapeshifter is kind of wasted on a Balance Druid since you'll stay in Moonkin most of the time to nuke stuff. You could probably find a better place for those 3 points.
-Imp Rejuv: Not a bad choice. Just remember that you cannot cast it while in Moonkin form. It will help when healing in instances if you ever have to do that.
That's it. Hope it helps a bit.