Spell Book: You now only have one rank of spell

Class Trainers: They have had an interface replacement

Multitracking: You may now track more then one object at time on your minimap. This is a huge benefit to you miner / herbalists

Professions Filter: Have had an interface replacement

Proc Watcher: The procs now have visual cues like power auras to remind you the ability is ready. Here is a screenshot of it in action.

Dressing Room: Zoom in feature added

Spirit Rez: A new Return to Graveyard button has been added for when you get your corpse stuck!

Pets: There is a new non-combat pet interface

Guild Menu: The guild interface has been completely redone

New Charachter interface

You will notice upon logging in that most of your gears stats has changed. Please re-enchant and gem your gear AFTER you figure out your new rotations.
Intellect = Spellpower

Spirit = MP5 for Healers, Hit for Shadow priests, Boomkins, Elemental Shamans

Agility = Attack power and Crit (Hunters, Shamans, Druids, Rogues)

Strength = Attack power and Parry (Warriors, Death Knights, Paladins)

Mastery = New stat only available through reforging at 80. It does something different for each class.

Hit rating has changed

Armor Pen is removed

Attack Power is converted to Agi or Strength

Spellpower is converted to INT on all items but weapons

Gem Changes: <--- Click for pewpew's article

Reforging: Reforging is a new system which will allow you to trade 40% of a single minor stat (haste crit spirit mastery) for another stat. This gives you more customization in your gear.

In the upcoming 4.0.1 patch, we'll be introducing a major overhaul of the current glyph system. This update features a host of changes including the addition of a new glyph tier, the transition of glyphs from consumable items to permanent spells, and a more streamlined user interface.

In its Prime
Glyphs will now be separated into three different tiers: minor, major, and prime.
•    Prime - Prime glyphs will typically provide direct increases to damage or healing throughput. Examples of prime glyphs include Glyph of Howling Blast, which will cause the death knight ability Howling Blast to infect targets with Frost Fever, and Glyph of Insect Swarm, which increases the damage of druid's Insect Swarm ability by 30%.
•    Major - Major glyphs will typically augment spells and abilities to offer additional utility. Examples of major glyphs include Glyph of Silencing Shot, which will cause a hunter to instantly gain 10 focus when successfully silencing an enemy's spell cast, and Glyph of Ice Block, which will reset the cooldown on Frost Nova every time a mage uses Ice Block.
•    Minor - Minor glyphs will typically provide players with additional convenience or cosmetic changes. Examples of minor glyphs include Glyph of Righteousness which will reduce the mana cost of Seal of Righteousness for paladins by 50%, and Glyph of Levitate which will remove the reagent requirement for a priest's Levitate spell.

In total, there will be nine glyph slots: three minor, three major, and three prime. As players level, they will be able to progressively unlock "sets" of these slots in increments of three (one minor slot, one major slot, and one prime slot). The first set of glyph slots will be unlocked at level 25, the second at level 50, and the final at level 75.

Teach a Man to Glyph...
In addition to the glyph tier changes, glyphs will also be transitioning from single-use items to permanent spells that a player can learn. While the item from which a glyph will be taught will still be consumed on use, once a glyph is learned, a player will always have access to it. This functionality will feel very similar to the current mechanic that allows players to learn certain recipes and patterns from item drops.

Despite these changes, activating a glyph will more or less function as it does now. To activate a glyph, players will simply need to select the desired glyph from their list of known glyphs and then click on the appropriate tiered slot.

Dust to Dust
With the transition from single-use items to permanent spells, the process for switching activated glyphs has been slightly altered. Similar to activating a learned glyph, players will need to select the new glyph from their list of known glyphs and then "place" it on top of the old glyph. While this process will not destroy the old glyph as it does at present, it will require a new reagent, Dust of Disappearance, which can be purchased from Inscription vendors or be crafted by scribes.

As a precaution, the following dialogue will also appear before a glyph is exchanged:

Are you sure you want to inscribe this glyph? The existing glyph will be lost.

Cost: 1 Dust of Disappearance

Players will be given the opportunity to confirm the exchange or cancel out of it. Please note that the old glyph will not be permanently lost; it will simply be deactivated.

Gotta Catch'em All
To complement each of these changes, the glyph UI window has been completely redesigned. All glyphs will now be organized in the glyph panel according to their tier (minor, major, or prime) and, much like how class spells and abilities display in the trainer window, will be sortable by "Already Known," "Unavailable," and "All Glyphs" categories. Players will also be able to search for glyphs using keywords and results will populate under each corresponding collapsible tier.

To access the new glyph UI, players will need to open up the Talent pane (default: N) and then click on the "Glyphs" tab.

Patch 4.0.1 will bring about major changes to all World of Warcraft classes, including sweeping talent tree revamps, altered spells, and improved abilities. We wanted to go over some of the more complex changes that are being made. If your class isn't listed here, it does not mean we didn't spend as much time on it -- the class will still receive a broad range of updates to talents, spells, and abilities -- we simply feel that these mechanics merit additional explanation.

Death Knights
Runes

The way in which runes regenerate has been changed. Rather than each rune type (Blood, Frost, Unholy) regenerating simultaneously, they now fill sequentially. For example, if you use two Blood runes, then the first rune will fill up before the second one starts to fill up. You essentially have three sets of runes filling every 10 seconds instead of six individual runes filling every 10 seconds. This allows the death knight rotation to be more forgiving, so that the player can take advantage of things like ability procs or respond to movement. This system makes it a little easier to get back on track without six separate runes now "out of sync" from the standard rotation.

You're a Bloody Tank

• The time has come to dedicate a death knight talent tree to tanking, and that is what is in store for the Blood tree. We felt that we could offer a more focused and polished tanking tree rather than having to scatter similar tools throughout the trees. This also allows us to really focus the other trees into more flavorful damage-dealing specializations.

Runic Empowerment

• This mechanic allows for runes which were on cooldown to instantly activate, which prevents the rotation from becoming too predictable and boring.

Druids
Eclipse

• This is a new mechanic being added for Balance druids. The resource is managed through a new bar near the health and mana bars. As the druid casts Arcane spells, the bar moves towards the sun element until the druid achieves a Solar Eclipse, which buffs Nature damage. The goal is then to cast Nature spells, until the bar moves toward the moon element, which triggers a Lunar Eclipse and buffs Arcane damage. The gameplay becomes trying to quickly move the bar from one end to the other in order to maximize each Eclipse buff.

Hunters
Focus

• This is a new resource which has replaced mana. Hunters never felt like they should be casting mana-based spells, and having a resource that lasted for a long time and then was painfully gone didn’t lead to compelling gameplay.

Pets

• Hunters can now use Call Pet to choose from among five different pets. They can swap out any of these five pets with other pets they have tamed by visiting a Stable Master. The Stable Master now allows the hunter to store twenty pets total.
• Pet families have been updated. Each family can now provide unique buffs to parties or raids, so if a class buff is needed for a raid and that class isn't represented in the group, chances are a hunter can provide a little assistance by calling out a specific pet.

Ammo

• Ammo is no more. As a hunter, you're just that good now.
• Any quivers or pouches used to store ammo will automatically be converted into a bag of roughly equal value.

Paladins
Holy Power

• This is a new resource which works similarly to combo points. Paladins will generally want to build up Holy Power until it shines through with a bright yellow glowing effect. Once Holy Power has been built up, it can be consumed to augment existing abilities. For example, Word of Glory can be used to cast an instant-cast free heal. This gives paladins several options for how to spend their Holy Power, depending on the scenario.

Warlocks
Soul Shards

• As items, Soul Shards are going away entirely.
• Any shard bags used to store Soul Shards will automatically be converted into a bag of roughly equal value.
• Soul Shards are now a new resource for warlocks. Each warlock will have a total of three. These Soul Shards can be expended using a new spell, Soul Burn, which allows the warlock to augment the next spell cast. The augmentation effect will vary depending on which spell is used, and all warlock spells which can be empowered by Soul Burn will have a tooltip listing of the effect it grants. Soul Shards will no longer be necessary for summoning demons or casting any other spell typically used when out of combat.

Warriors
Rage

• Rage is being normalized so that its generation is no longer based on damage done by auto-attacks. Each auto-attack will provide a set amount of Rage, with off-hand weapons granting 50% of the rage main-hand weapons do. Haste will allow Rage to generate more rapidly by increasing attack speed. In addition, while taking damage, the amount of Rage generated will now be based on the warrior's health rather than the opponent's level. The essence of this change is to improve the scaling that occurs with warrior gear. With warriors balanced around the top tiers of items, we've found that the class tends to underperform more drastically than others in lower-quality items. These changes should correct that.