This article will discuss the leveling of a priest for maximum speed (therefore optimal fun). The best way to level a priest is shadow. They deal the most damage under the least amount of time and take the least amount of damage. If you're going to level your toon solo for the most of the time like mostly everyone in the game, shadow is the way to go. If it is your plan to level up running instances constantly with friends, discipline (for the damage and healing capabilities) or even holy is for you (if you're the designated healer of the group). I will break down the leveling process into section of 10 levels:
You get access to talent points at level 10, and you should put your first points into Spirit Tap (And improved Spirit Tap) to begin your path down the shadow tree. This allows for mana regeneration after an experience delivering kill (or Mana Blast/Shadow Word: Death crit). Then place your next points into Improved Shadow Word: Pain and only three points into Darkness.
Get Mind Flay immediately. This move slows down your opponent and makes grinding much easier. It is then a no brainer to keep investing the points into Improved Mind Blast so that you can blast your opponents away faster and stop relying on non-shadow moves. The main focus here is to get to Shadowform as fast as possible. Once you reach level 16, it is wise to get Shadow Weaving. Shadow Weaving makes your more powerful as your attack mobs. As you grind your way towards 20, I recommend you go back and fill in your points in Darkness as well because of the increased dependency on Shadow affiliated moves.
Vampiric Embrace and Improved Vampiric Embrace. Heal yourself through DPS and regenerate mana through DPS, almost a grinding dream. Now you have a choice between Focused Mind and Shadow Focus. If you're having trouble hitting mobs, put your next points in the latter, otherwise the saved mana could help out in the long run. The next three points should be placed into Improved Devouring Plague for that extra periodic boost in health production while taking on a lot of mobs. Vampiric Embrace will stack with this attack.
Shadowform saves lives, literally. Grab this and watch your shadow power explode while minimizing damage. You should only be using shadow spells at this point anyway. Place your next five points to increase your critting abilities. Now you're beginning to feel like a caster. Improve your Shadowform for less disruption while casting as well as two points in Misery to help hit your opponents as well as increasing spell power during battles.
Snag Vampiric Embrace for that extra 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 agroing 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 aggroing 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.
To continue blowing away mobs without a break, Dispersion would be good for your toon. As well as that "Oh, Shit" button you can slam when there are just too many mobs to handle. Dispersion looks like a pvp talent, but it has its uses in soloing as well. Now its time to start heading down the Discipline tree to help you stay alive while managing your mana a little more efficiently. Put 5 points into Twin Disciplines to give your instant casts a little boost as well as make way for the next two more important talents: Inner Fire and Improved Power Word: Fortitude. These two talents give out a little bit more spell power and health for staying alive while bashing some monsters.
To finish off your massive grind on the way to 80, it is worth investing in Meditation for increased mana production while casting (stacks with Mana Tap). Inner Focus is a neat talent and acts like a power boost. I would recommend macroing this ability to Mind Blast for a very powerful Blast every once in a while (3 minute cooldown), as this ability does not trigger the global cooldown and can be used in congruence with other spells. That is as far as I would go into the disc tree, and the rest is up to you. Mind Melt and Twisted Faith are more damage boosting abilities to help boost your DPS.
I hope you enjoyed my Shadow Priest Talent Guide. I am sure there are other viewpoints out there, but experience tells me that Shadow is definitely the way to go if speed is your desire.