Snag Vampiric Embrace for that extra vindictus gold mana producing dot that will increase your damage and the duration in which you can do that damage.
After this, place the maximum points into Pain and Suffering to save some mana while you're burning a mob, but not in time for your Shadow Word: Pain to expire. This way Shadow Word: Pain continues to refresh and do damage as you Blast and Flay away.
After this is maxed out, go back and finish your points in Misery to unlock the next tier of points. The point of a leveling/grinding spec is to maximize damage for as long as possible. At this point in the game, it may be time to put some points in Crowd Control, as the major damage increasing talents have been used up.
I would personally recommend points be placed in Psychic Horror. An extra fear where the mob does not run away from you arguing other mobs is a priceless life saving talent. Mind Melt will help you more and more as you level and gear you find is more tailored towards a caster, or Priest specifically.
Putting points in that and Improved Psychic Scream will maximize damage the most efficiently while giving you that extra play room with fear if you're in trouble. If you're like me and you want to pick that Elite from behind some regular mobs without aggrading them, it may be overdue to invest in Shadow Reach. This gives you the distance to pinpoint and pick out the mobs you need to complete certain quests.
Most of the fighting you are doing as a solo leveling is one on one, with some random beating you over the head with his club or staff. This talent is not critical to leveling although it is a luxury.