Races, Racials and Talents
I will not be saying which race is best and which race isn't. I will be stating my opinion, you can easily go to 19 twink guides or look at the races for yourself and pick which race you think is better for being a 15 priest. I will be touching and explaining each race and racial in detail and how it benifits the 15 priest.
Alliance
Human: Every Man For Himself allows you to carry another secondary trinket while every other race will have to carry an Insignia to give them Freedom from stuns, fears, roots, movement impairing effects and so on and so fourth. This allows you to have another SP BoA Trinket, Haste BoA Trinket, or AGM. Mace specialization will provide you with an additional 1% spell hit which you need if you are going to be successful at self-peeling from those nasty rogues that try to gank or destroy you. The Human Spirit increases your over all spirit by an additional 3% which provides an overall increase in your mana regeneration.
Dwarf: Frost Resistance gives you a slight edge over frost mages as it reduces all frost damage that is done to you by 1%. It might not seems like much however this can make the difference between life and death in certain situations. Mace specialization will like the humans, provide you with an additional 1% spell hit which you need to be successful when self-peeling. Stoneform removes all Poisons, Diseases and Bleeds from you which is pretty much an anti-rogue ability.
Night Elf: Nature Resistance is great for those pesky shamans and druids who try to slow you down with totems or root you onto place. Quickness reduces the chance that melee and ranged attackers will hit you by 2% which can be game breaking as you are pushed up to level 19 so, it is treated as if you are a level 19 priest (granted; everyone should be hit capped for Night Elves at this point in the game), Shadowmeld allows you to go into the shadows and drop combat in order to escape, grab a quick drink, or confuse your enemy as you wait for Fear or Void Tendrils to get off CD. Wisp Form to the untrained eye might seem useless however; upon your first death that speed increase will get you back to your body faster than any other race and if you can get there before the GY Rez timer goes off. Your back in the fight before the respawn is.
Gnome: Arcane Resistance provides a 1% decrease in all Arcane Damage, great for those FOTM Doomkins and Arcane Mages. Engineering Specialization allows you to craft bombs and other goodies that can be used to your advantage. In example; you can craft a instant cast bomb to self-peel, or peel for a teammate, or a group of teammates. The only draw back to this is, you lose out on a Profession Slot and over time this profession can be costly. Escape Artist is great if you get rooted or slowed, it is also on a lower CD than the insignia so you can use them both without wasting either. Expansive Mind will provide you with a bigger mana pool than any other race (and if you are using +Mana enchantment to your chest, dear lord that mana). Shortblade Specialization can have its uses, however you will be primarily rolling with the PvE BoA Mace so you will not benifit from this unless you are rolling a
Dawnblade or the PvP BoA Dagger for a Stam/Flag Carry set.
Worgen: Aberration reduces all shadow and nature damage by 1%. This is a great talent against warlocks, druids and shamans. Darkflight is a mini-sprint/speed pot which allows you to get distance from you and your target, or an enemy or a group of enemies when self-peeling. Viciousness gives you an additional 1% crit, and if you are stacking crit. This is extremely useful, there is really nothing more I can say about this talent.
Horde
Troll: Beast Slaying DOES work on druids and hunter pets. This allows you to pump out an additional 5% damage compared to your regular targets who are not counted as beasts. Berserking increases your spell haste by an additional 20%, combine this with Lifeblood and Hurricane/Windsong Haste Proc and you can break the GCD point even quicker/easier than before. Da Voodoo Shuffle reduces all movement impairing effects by 15%, frostbolt, totem slows and other slows will be reduced, can't explain it any better lol. Regneration increases the regeneration rate increased by 10%. You also are allowed to continue to regenerate 10% of total Health regeneration may continue during combat. This is an interesting ability as you continue to gain health during combat, again this can make or break a fight but it is situational.
Undead: Cannibilize allows you to sit down in the middle of the battlefield and allows you to have a fine dining experience amist all the fighting. You rengerate 7% of your total health and mana over the duration of 2 seconds. You gain hp and mp back every 2 seconds and this only works on Humanoid Corpses so... you will have plenty to eat. Shadow Resistance reduces all shadow damage taken by 1%, again great for warlocks. Touch of the Grave allows you to drain health from your enemy target upon harmful spell damage. A little more healing towards yourself, it never hurts. Will of the Forsaken removes Fears, it shares a CD of 30 seconds with the Insignia however; you can use WoTF instead of your Trinket and save it for a Full Stun.
Tauren: Endurence increases your health by an additional 5%, inside of instanced battleground, this will definately give you more health than the average 15 priest and maybe even some 19 priests. Nature Resistance reduces all nature damage by 1%, another anti-druid anti-shaman ability. Warstomp is another self-peel that will stun anyone that is within 8 yards of you for 2 seconds. This can also be used offensively and defensively if you feel the need to peel for your FC if you hear him/her say {skull} SAPPED {skull} or something.
Blood Elf: Arcane Resistance Reduces all Arcane Damage taken by 1%, again a nice anti-aMage and anti-Doomkin. Arcane Torrent silences all enemies within 8 yards for 2 seconds and gives you back 2% of your mana. Pretty good if your playing hard O or hard D as you can stop all damage/healing for 2 seconds to allow a quick second for burst on the target(s).
Goblin: Best Deals Anywhere is no joke ability, it reduces vendor prices as if you were exalted. This allows you to possibly buy potions, scrolls and other misc items from vendors for dirt cheap. Pack Hobogoblin is great if you accidentally leave something in your bank, or need something from your bank as it is useable within battlegrounds. Rocket Barrage is an offensive ability that can be combined with your burst to deal some more damage to your target. As well as the more defensive ability Rocket Jump to push yourself a distance away from enemies. This is also great for juking from the roof to the FR back to the roof, second floor to second floor and many other areas. Time is Money is a great talent if you are going pure haste as you gain an additional 1% haste, you will literally hit the GCD point a lot faster with this as it is passive.
Neutral/Pandas
Panda: Bouncy cuts all Fall Damage by half, can be useful or a life saver if you fall from a very high distance or perhaps get elemental shaman AoE'd off a building. Epicurean allows you to gain double stats from all cooking buffs that you can eat which allows you to obtain 12 stamina/spirit instead of the regular 6 stamina/spirit from cooked food. Quaking Palm allows you to incap/sap your enemies which can cause a breif moment of rest allowing you to get a an ability off CD, save a teammate, e-bandage, or even a self-peel if used correctly. Granted; any damage on the target will remove the incap/sap so becareful with your DoTs.
Talents
Void Tendrils: This ability summons pretty much roots that will grab 5 targets (max) and hold them in place unless killed. I find this to be a key talent in the priest arsenal as it is a literal self-peel for you and your team. You can also bubble the tendril to keep it alive a lot longer because; generally when someone is stuck in a Tendril, they will either pop trinket which is foolish, or they will attack it and kill it, I also often times see a lot of people such as rogues simply stand in the Tendril and spam throw at me as I burst them down. I can't help but laugh when this does happen...
Psyfiend: Is a summon-able ally that you are also able to bubble (much like the Void Tendrils) however this does not root them into place. Every 2 seconds whoever is within 20 yards will be targeted and feared and re-feared. However; the Psyfiend will prefer anyone who is targeting and attacking you, so this talent does have its draw backs. However; it is pretty much a fear bot.
Mind Control: I guess this has its uses however I believe it is completely outshined by VT and PF. Regardless; you take control of your enemy target and can do what you want with them. This is a decent talent for taking people from the roof and throwing them back into the FR, perhaps grabbing the EFC and pulling him back to the offensive group. Again, this talent is completely outshined by the other two talents in the teir.
ain, Penance and Smite to go offensive with your cooldowns (Lifeblood and possibly Weapon Proc). The two DoTs can provide a lot of damage and keep stealthies out of the shadows so they cannot re-open on you or a teammate (i.e - Assassination Rogues). Even at 15 however; you can pump out some ridiculous burst damage as you can literally break the GCD point and go ham on an enemy. Sub-par 19 Twinking is something that has been completely turned upside down due to stat and gear scaling and I personally believe it can be viable for most, if not all classes.