Well, having a level 70 raiding priest, I feel I can contribute to this pretty well.
If you're going to want your priest to ever hit level 70, or at least before the game goes out of style, then you're going to have to go Shadow while leveling. As for a good shadow priest spec - just go full shadow, until you're like 65, or something, and can't put them into anything else useful, and in that case just put them into discipline. Shadow priests are fun, and wreck at high levels. They're useful in certain raids for crowd control, and high dot, as well as high dps. Parties love shadow priests too because of Vamp Embrace, and Vamp Touch, which give the party members hp and mana every time you do damage.
Once you're 70 though, it's generally assumed and requested that you're some variation of a holy spec. If you're in a cool guild, and the guild needs a little more dps for Gruul maybe, then they might allow you to go back to Shadow. As for holy priests, the talent trees can be tricky. This is where either you know your class, or you dont. Discipline is provided as a back up tree in regards to holy. A lot of the discipline talents are very useful when combined with holy talents. Since the holy tree isn't particularly good when gone pure holy, almost 99% of the good priests have their talents split between disc, and holy. I was 28/33/0. All raid groups will require atleast one of their priests to have improved spirit, which is disc, and which is why i went that far down on the discipline side.
My character:
http://armory.worldofwarcraft.com/character-talents.xml?r=Balnazzar&n=TimewastedSadly, though, I was very annoyed about the fact that Paladins are currently better at healing than priests. I was very good at my class, and had my talent tree perfected (although i often contemplated respecing to put 2 more points into my clearcasting, which procs a lot even only with 2% chance), but was still being out performed by paladins, as far as healing meters were concerned. Paladins were also put on main tank a lot, because they have much more hp and much more armor, not to mentiuon that can still put out a lot of dps. I was my guild's class leader and was a great raid healer, but I just feel like there should be certain things that should never be allowed, and a few of those should be that, A) nobody should be able to tank better than a good warrior and B) nobody should be able to heal better than a good priest. Thats just not how it is, though.
This is largely why I quit, but lately I've been wanting to create a new warrior. Once I get this job of mine going, and summer gets nearer, I'll prolly make a new warrior but until then, im stuck with my sad priest.
Edit: Also, don't make a priest unless you're looking for a class that honestly takes a lot of skill to play, and can be very very frustrating. My roommate even says priests look tough when he watches me in raid, and his main is a mage. A lot of people think it's either pressing 1, or 2, but they're 100% wrong. Priests are hard.
