Account-wide achievements, pets and mounts All pets, mounts, and achievements will be account-wide as of patch 5.0.4. This feature is Battle.net account-wide -- if you have two WoW licenses under one Battle.net account, your pets, mounts and achievements will be shared across your characters on both licenses. Please note that the account-wide system does not include reputation. For example, if you have exalted reputation with Therazane on one character, your other characters will still have to grind it out.
AoE looting AoE looting will also be rolled out with patch 5.0.4. Gone are the days of clicking corpse after corpse. As of 5.0.4, you'll be able to simply click one glittering corpse out of the giant pile to loot all of them at the same time. And your computer mouse will breath a quiet sigh of relief
BattleTag support If you have been playing Diablo III at all, you've noticed the BattleTag system, which works similarly to Real ID but does not include real names. Right now, you can only add and remove BattleTag friends in Diablo III. With patch 5.0.4, this system is being placed in WoW as well. As of patch 5.0.4, you'll be able to add, remove, or invite BattleTag friends in the exact same manner as you do now with character name or Real ID.
Class and talent system changes All class and talent changes that have been going on in the Mists of Pandaria beta will be available in WoW with patch 5.0.4. Because of this, all players will have their talent points refunded upon login on Aug. 28. Many of the old talents have simply been rolled into new character abilities, and new spells will be learned automatically, without having to visit a trainer. For druids, this means that your fourth class specialization, Guardian, will now be in effect.
Item changes Head enchants will be a thing of the past, as will relics and ranged and thrown items. All weapons will now be equipped in the weapon slot. For hunters, this means you'll have your bow or gun in your weapon slot. For casters, this means that you may now equip a wand as a main-hand weapon. All ranged weapons will be updated to be more powerful as a result of this change, and ranged weapons no longer have a minimum range. In addition, spell penetration has been replaced by the new PvP power stat on existing items.
Profession changes The only major profession changes we'll see in 5.0.4 are glyph changes. Prime glyphs have been removed, and many glyphs have been changed, added, altered or moved to different glyph types.
UI changes Your UI is going to look very different upon logging in. Spellbooks have been updated to reflect changes, buffs have been consolidated in the UI, and the pet and mount tab now has its own user interface. In addition, character creation screens will be updated, and a new roll results frame will be added. The dungeon journal has been expanded to include information on all pre-Cataclysm encounters. And just in case this is confusing, a new help system has also been added to many frames.
World Event: Attack on Theramore Isle This world event will begin once patch 5.0.4 hits. At the present time, we really don't have much information on how this event is going to play out. Testing for the event has been kept internal. However, we can probably expect a new series of quests, a new achievement, and possibly a new set of rewards from the event.
Pet battles While pet battles will be available even to players who have not purchasedMists, the feature will not roll out until Mists launches in September.
Monks Players will not be able to roll the monk class until Mists of Pandaria launches.
Pandaren characters Pandaren characters won't be surfacing in Azeroth until September.
New/heroic dungeons New dungeons will not be available until September.
Pandaria Needless to say, you will not be making any trips to Pandaria until the expansion launches