**Some of these glyphs are changing in patch 3.3. I'll update the guide shortly. Glyph of mindflay is now awesome, not much else to note.**
While you're busy grinding your new shadow priest to 80, I'll save you the trouble of researching glyphs and give you your options here. It won't take me long, there aren't many. Seriously, I don't know who designed the array of priest glyphs but they a) actively hate the class, b) have no clue about the class or c) are a moron. Take your pick.
We're going to do this by level, as our glyph slots open up. It's not a terribly logical arrangement but see paragraph above. Sometimes it makes sense to switch glyphs around in-between, and I'll mention those too at the appropriate level. If you're really interested, I have listed the whole gamut of priest glyphs at the bottom, along with the level they become available, so feel free to ignore me and plan your own spread.
Level 15 — one major, one minor
Congratulations, your first glyph slots! At level 15, you have access to seven major and two minor glyphs.
The selection of uninspiring major glyphs include a couple you should never consider under any circumstances (inner fire, fade), and two that you can't even use yet (shadow word: pain, smite). At this level, you will probably be shielding a lot, and the glyph of power word: shield is one of the few decent options available.
You only have access to two minor glyphs at this stage and one of them is worthless. Take the glyph of fortitude.
When you learn inner fire, you might want to pick up the glyph of smite. The effect was changed recently from reducing pushback to increasing damage while the inner fire dot is on the target (thanks to Aaron in the comments for pointing this out). Personally, I took smite off my cast bar at 20, too irritated with the long cast time and mana drain, but even if you cast smite only once per target this glyph will do more for you than power word: shield.
Level 30 — one major
Now is the time to pick up the glyph of shadow word: pain. If you really want to, you can pick it up sooner to replace your first glyph, but as I mentioned in the levelling guide, mind flay is not instantly a great spell. It takes some time and some additional talents to really show it in its best light. It's not really worth a glyph until level 28 when you pick up rank 2 of the spell. Your best options for the second slot remain power word: shield and smite — which you choose really depends on how often you use smite.
Several additional glyphs have opened up to you by this level, but I wouldn't advise any of them for levelling at this stage. They include a number of healing glyphs, and some dubious 'utility' glyphs. Only two are true dps glyphs. The glyph of shadow, which opens up at level 20, isn't usable until at least level 40 (see what I mean about the design?) and the glyph of mind flay trades the spells snare for reach, which is a poor deal for levelling.
In a rare moment of clarity, the designers made one glyph available at the same time as you learn the affected spell — levitate. I love the spell and I love the glyph and happily recommend you swap out the glyph of fortitude for it. It's clearly not mandatory though — light feathers aren't that expensive or hard to come by, so you may choose to keep the reagent cost instead.
If you have spent your talent points wisely, you should now have access to shadowform. Go ahead and equip the glyph of shadow, replacing either the glyph of power word: shield or smite, whichever you've been using alongside shadow word: pain. At your level of gear, it won't provide a huge boost, but it will do more for you than any other glyph in the game after shadow word: pain. Sadly, you now have the two best glyphs for raiding; there's nowhere to go from here but downhill.
Level 50 — one minor
In the absence of any brilliant new glyphs, your best bet is to go back to fortitude. Glyph of shadow protection isn't a bad option for raiding, but I don't remember casting it often while levelling, and certainly not to the extent that the duration bothered me. Think about switching these glyphs around when you start raiding.
Level 70 — one minor
This is as good a time as any to equip the glyph of shadowfiend. It's the closest thing we've got to a mandatory minor glyph and you'll want it at 80 anyway.
Level 80 — one major
Ok, so you're not strictly levelling anymore but I'll cover off the last slot just to be thorough.
If you're going straight into raiding, I urge you to seriously think about the glyph of mind flay. Yes, it is an outrageous abuse of the glyph system to solve a fundamental flaw in a spell in this way, I hate that we are effectively cheated out of a slot, but I cannot stress how beneficial the added range is. There are many raid fights where positioning for an un-glyphed mind flay is very difficult and mind flay is too important to your raid dps to risk ever being out of range. Having said that, if you are confident you can manage the range issue without a glyph, you have a couple of other options. Glyph of shadow word: death provides a kind of ghetto execute, but don't expect this spell to contribute significantly to your raid dps. Glyph of mind sear is a fantastic choice for trash, but less useful on many bosses.
If you're going to continue questing and focus on solo content for a while yet, skip mind flay for now and consider either shadow word: death or dispersion for the shorter cooldown.
Full list of major glyphs
Numbers indicate the required level.
Glyph of Fade (15)
Glyph of Inner Fire (15)
Glyph of Power Word: Shield (15)
Glyph of Psychic Scream (15)
Glyph of Renew (15)
Glyph of Shadow Word: Pain (15)
Glyph of Smite (15)
Glyph of Dispel Magic (18)
Glyph of Fear Ward (20)
Glyph of Flash Heal (20)
Glyph of Holy Nova (20)
Glyph of Mind Flay (20)
Glyph of Scourge Imprisonment (20)
Glyph of Shadow (20)
Glyph of Mind Control (30)
Glyph of Prayer of Healing (30)
Glyph of Spirit of Redemption (30)
Glyph of Lightwell (40)
Glyph of Circle of Healing (50)
Glyph of Pain Suppression (50)
Glyph of Dispersion (60)
Glyph of Guardian Spirit (60)
Glyph of Hymn of Hope (60)
Glyph of Penance (60)
Glyph of Shadow Word: Death (62)
Glyph of Mass Dispel (70)
Glyph of Mind Sear (75)
Full list of minor glyphs
Glyph of Fading (15)
Glyph of Fortitude (15)
Glyph of Levitate (34)
Glyph of Shackle Undead (20)
Glyph of Shadow Protection (30)
Glyph of Shadowfiend (70)
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