So, after leveling a lot more, I'm reconsidering some of my spec choices from earlier. While I don't have to re-spec yet, I'm shuffling some priorities around but still proceeding down the DISC tree.

The big change so far is putting 2 more points into Shadow (total of 7). Which gives me Spirit Tap (5) and Improved Shadow Word: Pain (2). The Spirit Tap ability gives me +50% mana regen after an XP giving mob dies (solo or a 5-man group, doesn't work in raids). And the Improved SW:P points boost the duration of my primary DoT spell by 30% (base is 18 sec, boosts to 21 sec then 24 sec).

What I was finding when soloing is that my SW:P runs out earlier then I'd like. Normally, I cast 2 nukes, one DoT, and my shield spell (PW:S) at the start of a fight, then use my wand for damage until the mob is dead. This keeps me well outside the "five second rule" (FSR), which means that I'm spending the second half of the fight getting mana back. On a good fight (mob within +/- one level), I can end the fight at around 80-90% mana, and the Spirit Tap buff that runs for 15 seconds will fill me back up (or close) before I pull the next mob.

However, because higher level mobs take longer to kill, SW:P was running out before the mob was below 10% health. Which means that I have to make difficult choices. I can either stop wanding and re-cast or try the wand the mob to death. If I recast spells towards the end of the fight, I put myself back inside the FSR box and I'll end the fight at 50-70% mana. If I don't re-cast, I'll end up at 50-70% health (instead of 85% or higher), which means I'll have to recast my armor buff and probably a heal.

So, hopefully, taking the 2 points in Improved Shadow Word: Pain, I'll be able to stick with my normal solo style of frontloading all of my spells in a burst, then simply wanding until the mob is dead. Which is a spec that looks like this.

Upcoming talent picks (up to level 44):

Mental Agility (5 points) - 10% less mana on insta-cast spells
Divine Spirit (1 point)
Improved Divine Spirit (2 points)

Level 45 to 59:

Divine Fury (5) - lower cast times
Holy Spec (5) or Spell Warding (5) - not sure which...
Improved Healing (3)
Holy Spec (2) or Spell Warding (2) - not sure which...

For level 60 to 70:

Spiritual Guidance (5)
Spiritual Healing (5)
Spirit of Redemption (1)

At which point, I will probably un-spec the 7 points in Shadow.

Healing Prayers (2)
Holy Concentration (3)
Mental Strength (2)

Which puts me at 25 pts in DISC and 36 pts in HOLY. It's a bit of a hybrid build, but should be pretty effective in a wide variety of situations.