Enchanting materials are good, right? Well, not so much when you could earn more gold from selling what you disenchanted than it would cost to buy the result of the disenchanting.
To go into more details:
BoP blues from instances – especially Wrath-level blues as well as the blues from the starter Cataclysm instances, which disenchant into Small Heavenly Shards rather then normal ones. So you’re taking a weapon that vendors for 20g+ and turning it into a 5g shard? Well done you.
BoE blues from instances. As a rule of thumb, I never roll “Disenchant” on these. Why? They often sell for vastly more than the shard would… and if they don’t, it’s easy enough to get them disenchanted anyway, since they’re BoE. Granted, I’m a little bit spoiled, since my main is an enchanter and I don’t have to ask anybody else to shard things for me… but still.
BoE greens require a bit of thought, as the market for them is highly variable.
Vanilla greens are often worth disenchanting – especially the higher-level ones that disenchant into Greater Eternal Essences and Illusion Dust; keep an eye out for well-statted low-40s plate and mail, though, as people will often pay a premium to kit their alts out in plate and mail once it’s available to them.
TBC greens are worth disenchanting across the board, except for sub-60 greens with excellent suffixes, as these can sell very well (although the market’s shrunk a bit since Cataclysm, since the remodeled sub-60 quests give excellent gear rewards).
Wrath greens will most likely be more profitable sold to a vendor this early on in Cataclysm due to the high availability of Wrath enchanting materials.
Well-statted sub-80 Cataclysm greens should never be disenchanted. The two Hypnotic Dust or Lesser Celestial Essence you will get from disenchanting those will not make up for the 100g+ each those greens are worth on the AH. I cringe every time I see one of those get disenchanted in an instance.
You don’t even need to memorize what items disenchant into what materials. Get an auction addon (Auctionator or Auctioneer), do regular scans and the information will be right in the item’s tooltip.