Cataclysm has simplified many secondary character stats into primary stats. Here's how to figure out if the next piece of gear you come across is a good fit for your character:
Stamina: More stamina means more health points. Stamina comes standard with most gear. Unless you're a tank, you don't normally need to pick Stamina over any other stat.
Strength: Attack power for plate-wearers: Paladins, Warriors, and Death Knights.
Agility: Attack power for leather and mail-wearers, and melee critical strike: Rogues, Feral Druids, Hunters, and Enhancement Shamans.
Intellect: Increases your mana pool, spell power, and spell critical strike: Warlocks, Mages, Priests, Holy Paladins, Elemental & Restoration Shamans, Balance & Restoration Druids.
Spirit: Increases your mana regeneration rate. Unless you're a healer, you don't normally need to pick Spirit over any other stat.
Haste: Increases your melee attack speed and spell casting speed.
Critical Strike: Increases the chance you will land critical strikes with melee attacks and spells.
Hit Rating: Reduces your chance to miss against mobs that are higher level than you.
Mastery: Gives a bonus to your talent-specific Mastery ability (requires level 80).
Parry: Increases your chance to parry enemy attacks. (Paladin, Warrior, and Death Knight tanks want this.)
Dodge: Increases your chance to dodge enemy attacks. (Great for all tanks including Feral Druids.)