I've now hit 50, so few more words of experience:
1) Focused Casting is pretty much useless in instances. Yes, even in those OH SHIT moments, because if you're getting critted by elites, you're either going to die in few seconds or the party is fucked anyway. However, while soloing it is quite useful when you accidentally pull too many mobs, your PWS is on cooldown and HP bar is plummeting. For same reason, Healing Focus is useful for keeping you alive. I've lost count how many times those two talents have saved my life. So for levelling, Martyrdom beats IPWF, imho.
2) Inner Focus and Meditation are both very useful in instances and when soloing, probably everyone agrees.
3) Once I got 5 points in Holy Specialization and 2 points in Searing Light, I saw a very nice increase in my damage output. Obviously my DPS is still quite weak, but I can keep killing mob after mob after mob after mob and killing each one doesn't take too long (though enviously I watch mages kill their mobs with 2 fireballs)
4) Holy Nova has come useful in Uldaman, Maraudon and Sunken Temple, as all three instances have situations where a bunch of non-elite mobs swarm the party. Keeps everyone alive and does some damage. Holy Reach helps there as well. Funnily, I've been the puller in some instance runs thanks to the extra distance that Holy Reach gives me, when there hasn't been a hunter or a mage present. But I haven't used Holy Nova while soloing and the extra range of Holy Reach isn't that useful when you're alone either. Still, twice I've finished off a Hordie trying to run away thanks to that range, so...
So while switching to Shadow at 40 makes for more efficient levelling, at least damage out-put wise, remaining with the talents that I linked to in the OP is also viable. Plus, I've made tons of friends thanks to good healing in instances, so finding groups is very easy. Or maybe that's just the high population on the server
