One of the changes that will have a pretty significant impact on how we play both in PvE and PvP is that many buff and debuff spells are being changed or removed. It can all seem a bit complicated at first viewing, what with all the class specific changes, but it's really fairly easily broken down and it won't directly affect how you play all that significantly. We're going to go over the changes and explain what they are and how they'll change how you play, if they do.
First up is the Weakened Armor debuff. Essentially, this is the old-school Sunder/Expose Armor affect. It's gone entirely. Rogues no longer have Expose Armor, warriors no longer have Sunder Armor and the Devastate prot warrior ability no longer applies Weakened Armor. In addition, the druid ability Faerie Fire now applies Physical Vulnerability instead, increasing all physical damage taken by its target by 4% for 30 seconds. This is because it was thought that having both Weakened Armor and Physical Vulnerability was excessive, since they did basically the same thing. What this means in terms of how we play? Almost nothing. If you're a rogue, you won't use Expose Armor anymore because it's gone. A warrior tank will hit Devastate the same as they always did. Druids are still going to use Faerie Fire to debuff things. Hunter pets that had similar abilities will no longer have them.
Next up is the Weakened Blows ability that almost all tanks had - it reduced incoming damage. The abilities that provided it still exist, they simply don't provide it anymore. Thunder Clap is an example of an ability that currently provides the debuff, but won't in Warlords. Since all tanks applied this debuff, it basically wasn't very meaningful, and instead monsters will simply be tuned to do less damage to compensate for its removal. What changes for players? Nothing. You won't apply the debuff, but you'll still use those abilities that did once apply it.
Next up is the Cast Speed debuff. It's mentioned that it was primarily used in PvP, and helped contribute to the need for instant cast spells and heals, since players would often find themselves debuffed and their heals taking longer. With the other changes to healing and cast on the move spells, it's not necessary and was removed. For players, this means that death knights and mages no longer apply this with their Necrotic Strike and Slow abilities, and rogues lose Mind-Numbing Poison while Warlocks lose Curse of Enfeeblement. This will mean a little adjustment, especially in PvP, but with so much else changing there it's just one more thing to adjust to.
Next up to consider is the consolidation of all spell and attack speed buffs into flat out haste buffs. Previously there were a variety of buffs that buffed one or the other of these, and now, they'll all be basically the same thing. This primarily means that you won't need to have one class to provide it for melee/hunters and another for casters - one player with the buff can provide it for everyone. So now Balance Druids Moonkin Form, Shadow Priest's Mind Quickening, Elemental Shaman's Elemental Oath, Enhancement Shaman's Unleashed Rage, Frost and Unholy DK's Unholy Aura, and Rogue Swiftblade's Cunning will all provide a flat 5% haste. The only real change here is that you won't need two people to have a speed buff to cover everyone.
Finally, there are a few other changes. Blood DK's get a flat mastery buff - Power of the Grave: grants a bonus to Mastery to all Party and Raid members. Brewmaster monks gain Legacy of the White Tiger, granting 5% increased Critical Strike chance to all Party and Raid members. And shadow priest Mind Blast adds 5% additional magic damage taken for 15 seconds, as does Rising Sun Kick from windwalker monks.
Taken all in all, these significant changes don't have tremendous learning curves - either you don't change how you play at all, or you simply stop using an ability because it no longer exists, with a few additions that are fairly self explanatory. So if you've been worrying about it, you can breathe easier knowing it won't really affect your gameplay much. The full buff/debuff section of the patch notes is reproduced below for your perusal.