It's also worth noting that almost every slot with an enchant has a weaker version and a more powerful version. Although it's usually frowned upon to go weak when can go strong, in the case of enchants, it's probably okay at the entry level to go with the weaker version of an enchant. The more powerful ones generally take Abyss Crystals and the like, and may be a bit steep. In addition, there's a few other quirks and special enchantments to watch out for. Let's take a look, by slot, at some of the best choices for enchanting for both DPS and Tanking.
Runeforging is better than enchanting, just to get that out of the way. There's no use wasting shards and dust on a weapon enchant when a better one is just a Death Gate away. As far as which to get, though, let's look at the options.
For DPS, it's very hard to go wrong by just applying Rune of the Fallen Crusader. The health will help soak up environmental damage, and strength will translate to AP, which will translate to spellpower, which means a large overall boost to your damage. Even better is the fact that it scales, meaning the better your gear gets, the better the proc gets. If you're running a Howling Blast build, you might get away with Razorice (Some of our commenters suggest using it on the offhand while keeping crusader on your main hand), but otherwise, there shouldn't be any question in your mind that Fallen Crusader fits the bill.
Tanking has a choice between two enchants: Rune of Swordshattering and Rune of the Stoneskin Gargoyle. If you haven't hit the 540 skill defense cap, take the gargoyle, no ifs, ands, or buts. Once you get past 540, there's some small amount of debate over which enchantment works best. 4% parry is a very nice chunk of avoidance. On the other hand, the 2% stamina on the gargoyle rune is still pretty nice, it works from all directions (whereas parry works only from the front) and even past 540 defense, you do get some avoidance. In general, if you feel like your stamina is a little low, or you want to unload some stamina for threat on your armor, you can use Rune of the Stoneskin Gargoyle to pick up some of the slack. Otherwise, that 4% parry will probably prove superior if you can keep your enemies in front of you.
For your head, DPS will want to pick up the Arcanum of Torment from the Knights of the Ebon Blade. For Tanking, you'll need revered Argent Crusade reputation for the Arcanum of the Stalwart Protector. It's a nice and straightforward choice, which is always cool.
Much like the head, here's another straightforward choice. Grind Sons of Hodir reputation, and at honored you can grab the Lesser Inscriptions of the Axe or the Pinnacle, depending on your role. At exalted, grab the Greater versions. Of course, you can also level inscription and skip the whole mess altogether, but otherwise, it's definitely worth the grind.
For tanking, you can grab some quick and easy defense via the Greater Defense enchantment. DPS or tanks who don't need defense can grab Super Stats, or if they have the gold to spare, the more powerful Powerful Stats.
On Cloaks, Titanweave is a great place to pick up some defense rating for a Tank. You can also grab Agility or Armor if your defense rating feels solid enough.
For DPS, the choices are pretty scarce if you're not a tailor. You may just want to go with an agility enchant for a bit of extra crit, or go for Wisdom for the threat reduction.
For tanks, Major Stamina is amazing if you have the gold to spare. It's steep though, so if you can't afford it, you can probably get by with Greater Stats until you get more gold or some bracers worthy of Major Stamina. Of course, if you're desperate for a last few points of defense, you can track down a Burning Crusade era enchanter with the Major Defense enchant.
For DPS, you'll want to track down either Greater Assault or Striking, depending on how much gold you want to spare.
Tanks will probably want to go for Armsman. The Eternal Earths really aren't that hard to come by, especially if you're a miner, and 2% threat and a bit of extra avoidance is hard to beat. Of course, if you're absolutely secure in your ability to keep threat, you could always grab Expertise to get closer to pushing parries and dodges off the table.
For DPS, you can use Precision to fill some of your hit rating needs. If you're fine on hit rating, go for Greater Assault or the more powerful (and still relatively affordable) Crusher for the attack power.
There's no enchantments for belts per se, but be sure to grab an Eternal Belt Buckle. It's perfect for, say, a Bold Scarlet Ruby if you're DPS, or a Solid Sky Sapphire if you're a Tank.
You'll need to seek out a leatherworker here. The epic leg armor used to be a clear choice, but now that it requires Frozen Orbs, you may want to just stick to the blue-quality stuff until you get Naxxramas level legs depending on your cash flow. For Tanks, you'll want epic Frosthide Leg Armor, or the cheaper Jormungar Leg Armor. Both provide plenty of stamina and some added agility for bonus avoidance. For DPS, you have the Icescale Leg Armor or the cheaper Nerubian Leg Armor.
Tanks here can pick up a nice chunk of health with a Greater Vitality Fortitude enchant here. Of course, if you don't have Unholy Aura, you may also want to consider Tuskarr's Vitality. A bit of extra speed can help in wrangling and positioning mobs more than you might expect.
DPS can grab Icewalker here for a quick and easy hit rating boost. If you've hit 8% hit though, you're better off going for the attack power enchants. Greater Assault isn't too steep in price, but you can also get the Assault enchant for even cheaper.