- If you've been using various leather or mail pieces with attack power, said AP is about to drop off of them. There will be no attack power on gear of any kind. Attack power will purely be converted from either strength or agility depending on class. This has been said before but it bears repeating. If gear currently has AP on it, that AP will convert to agi and stam.
- Armor penetration on gear will also be going away. Current gear with armor penetration will see that stat changed to haste or crit. (So if you picked up an agility cape because it had AP and armor pen on it, it's going to change pretty radically and you may want to look for a strength cape.)
- Haste is being changed to make it less of a 'oh lord not haste' stat. Since the description is 'Haste will become more attractive for melee classes by allowing them to recover resources such as energy and runes more quickly. Our intention is for Haste to let you "do stuff" more often' and that doesn't seem to fit how rage is generated, we can most likely expect to see a pretty radical redesign of rage generation. If not, haste will still not be all that great for us.
- Mastery is incoming. What will mastery do? Well, we don't know yet, exactly. "It's directly tied to talents, so what you gain from improving this stat is entirely dependent upon your class and the talent specialization you choose." We can speculate that perhaps it will make our Bloodthirst hit harder or our Mortal Strike refresh more rapidly, but that's all just speculation: there's no specific details yet on Mastery. I just made those two up. Please don't get mad when they don't happen, I'm fragile.
- Ratings in general will be harder to 'cap out' so things like hit or expertise on armor will be good longer. You won't get exactly X amount of expertise and then stop and find yourself raiding three tiers later and saying "I'd love those pants if not for all that wasted expertise on them" as an example. Also, as you progress in raids, monsters and bosses will be getting 'harder to hit/crit' meaning that higher ratings will be needed to damage them.
- Plate DPS gear won't provide such a large stamina bonus to plate DPS classes. (That's you.) Non-plate wearers will catch up. Similarly, plate won't offer as much of a mitigation advantage to plate DPS classes. You'll still hear 'but you wear plate' from rogues, shamans, druids, mages... heck, everyone who doesn't wear plate. But it won't be as significant, so in addition to laughing at them because armor doesn't mitigate poison or fire or whatever, you'll laugh at them for not knowing that plate isn't as good post Cataclysm.
- Resilience will only work against players and will only reduce incoming damage and critical damage. You won't be slapping on your PvP gear to offtank anymore.
World of Warcraft: Cataclysm will destroy Azeroth as we know it. Nothing will be the same. In WoW.com's Guide to Cataclysm you can find out everything you need to know about WoW's third expansion. From Goblins and Worgens to Mastery and Guild changes, it's all there for your cataclysmic enjoyment.