Obvious Things to Know When Playing A Priest

1. Haste effect from Power Infusion does not stack with Heroism / Bloodlust.
2. Power Infusion can be used on others.
3. Twin Discipline affects Smite and Flash Heal triggered by Surge of Light. It also affects Mind Flay and Penance as an exception. It also increases Power Word: Shield (PW:S) absorption amount as well as healing done from Glyph of PW:S
4. Inner Focus + Divine Hymn = ZOMG!
5. Improved Inner Fire not only increases armor value granted by Inner Fire (IF), it also increases the total spellpower gained from IF by 54, giving you a total of 174 spellpower from the IF buff.
6. Aspiration reduces the cooldown of your Power Infusion, Pain Suppression, Inner Focus, and Penance spells on a percentage base. This means the shorter the cooldown, the smaller the reduction. For example, if Penance has a 10 secs cooldown, it will be reduced to 8 secs by Aspiration. If Penance has a 8 secs cooldown from Glyph of Penance, it will be reduced to 6.4 secs, -1.6 secs by Aspiration.
7. Pain Suppression is not bullet proof. It only reduces damage taken by 40%. You can still die when under the effect of PS so heal yourself!
8. Spell Warding reduces all magical damage taken. This includes curses, spells, fire/frost/nature/arcane/holy/shadow boss damage.
9. Desperate Prayer can saves raids. Dead priests can't heal. So spend a point for your own good.
10. Spirit of Redemption doesn't last that long. Evil GCD! You can cancel the effect by right click on the SoR icon on your buff bar.
11. Warlocks love Lightwell, doesn't mean you will spec into it, but still, warlocks love it so spread some love.
12. Guardian Spirit + Your Faction Leader = Angry Opposite Faction Raiders
13. Power Infusion + Prayer of Healing = ZOMG! And you think discipline sucks at aoe healing? (well, they do comparing with holy, but they don't suck as much as you think they do)
14. Dispersion is not bullet proof if you cast it at 5% health
15. You have a buff called Shadow Protection. It's one of those things you can use to not die on a certain Voidwalker in Violet Hold.
16. You have a spell called Power Word: Shield. It prevents spell pushbacks, it can be cast on others. It doesn't matter if you are discipline, holy, or shadow. Use it, love it, it saves lives.
17. Mind Control + Opposite Faction Players + Arathi Basin Lumber Mill = Death
18. Levitate + High Grounds = Soft Landing
19. Levitate can be used on others
20. Spirit Tap and Improved Spirit Tap do not stack
21. Spirit Tap is great for leveling.
    
22. Cure Disease, Abolish Disease, Fear Ward, Shackle Undead are all castable in Shadowform
23. First Aid does not cost mana, so learn it, use it, and love it. Especially if you play shadow.
24. You have a spell called Mind Vision. It can be used on friends or foes. It cannot be dispelled. Rank two does not require Line of Sight. Cast Mind Vision on rogue or druid before a duel will prevent them from going into stealth. You can also target flag carriers using Mind Vision, a wonderful spy glass. (Just keep an eye on your own health, while your mind is away in another's, your body is still in a war zone.)
    
25. You can break Polymorph by timing your Shadow Word: Death to land at the same time as Polymorph to break cc.
26. You can Dispel.
27. You have a spell called Binding Heal, it heals both yourself and your target for an amount equal to a Flash Heal. Consider it as a double Flash Heal that takes less time to cast and cost less mana with a lot less threat.
28. You have a spell called Mind Soothe, it's great to use on targets before Mind Controlling them. But be careful as it can be resisted. When that happens, be prepared to run as you will pull a whole pack of mobs. Also, Mind Soothe animation look exactly like Sap, so toss this around in battleground to cause some serious confusion, enough to keep your target starring at the screen for a fraction of a second or so. It might just be enough to get you out of a tough spot.
29. As a priest, half of your spellbook is filled with healing spells, so use them. Even if you are shadow, you can still heal in emergency situations.
30. If you apply Shadow Word: Pain before x5 stacks of Shadow Weaving, you need to reapply it yourself after reaching the 5 stacks. SW:P refreshed by Mind Flay will not carry on the characteristics of the new 5 stack SW, causing you to lose a good chunk of your dps.
31. Glyph of Smite is excellent for leveling.

The list goes on and on, there's probably a lot more obvious things but I just cannot remember them at the moment. o.O"