Intellect - Intellect is incredibly useful to discipline priests more than other healers because in addition to providing spellpower, our primary mana regeneration mechanic, rapture, is also tied to our maximum mana pool, which is affected by intellect. Other abilities that restore mana based on maximum pool size are shadowfiend, hymn of hope, archangel, and replenish.
As a result, gemming should always aim to include intellect where possible. If a slot can take a red gem, get intellect. If it takes a yellow gem, get an orange gem that includes intellect. If it's blue, get a purple gem that includes intellect. The most effective meta gem for discipline priests is also the ember shadowspirit diamond. In addition to providing more intellect, it increases our mana pool size by another 2%.
Spirit - Spirit will provide a significant amount of mana regen, and as such, every item you pick up (except for trinkets) should have spirit on it, all other things being equal. This is to say, you should never have to reforge another stat to spirit. Obvious exceptions include large jumps in iLevel that provide enough of a boost to other stats (such as intellect).
Crit, mastery, haste: There is no right answer for all discipline priests to take as far as these stats are concerned. It will largely depend on your playstyle and raid composition.
If you are casting a lot of power word: shield, then mastery is going to be your most valuable stat.
If you are casting most of your spells on a regular basis, then crit will be your most valuable stat.
If mana is not a concern, and people are dying because you aren't healing them fast enough, then haste is your most valuable stat.
Effectively, haste increases your throughput at the sacrifice of efficiency. The longer a fight takes, the less attractive haste becomes since it will run your mana dry. The shorter a fight is, the better haste becomes. Typically, haste will be the least-valued stat at early gear levels and becomes more attractive as you gear yourself up.
Mastery will only affect divine aegis and power word: shield, so for all other spells, mastery is not-quite-but-very-close to worthless. Now that prayer of healing's divine aegis can also benefit from crit, it also means that mastery is only more valuable than crit as far as power word: shield is concerned. However, power word: shield, as of patch 4.0.6, is now our primary go-to heal.
Again, whether or not you want to stack crit or mastery is based largely on just how much you're using power word: shield. There are priests completing top-end content such as heroic raids who will stack either one, so it's dependent on how you use your spells.
As far as socket bonuses are concerned, anything that provides more intellect should always be gemmed for when possible. Anything that provides spirit as a socket bonus is typically also very beneficial to gem for. Blizzard has, in general, tried to make socket bonuses much more attractive to use, and they should be since all 5 stats are now important to discipline priests.
There don't seem to be a large number of enchants for any given item slot that healers can use as of yet. As a general rule of thumb, however, grab intellect where possible. Otherwise, grab spirit where possible. Otherwise, grab whichever stat you are currently low on.
Penance is mandatory, as it is our most efficient heal, and will heal for a great deal, as well as stacking up grace to its full capacity. Power word: shield is also a spell that make frequent appearances in our rotation, so power word: shield is a must.
Use whichever you feel comfortable with.
Divine accuracy is mandatory for smite specs, as it will give you a guaranteed chance to land the spell whenever it's cast. Prayer of mending is also an attractive option. All other major glyphs are up to playstyle. I wouldn't recommend the glyph of smite because holy fire will often miss, and the DoT from it simply doesn't last long enough to take advantage of.
Mass dispel comes highly recommended, as the times you'll need it, you'll want it to go off as fast as possible.
Shadowfiend and levitate come recommended, but these are all also up to personal playstyle.