Here are the details – straight from a Blizzard Employee’s post on the WoW forum:

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.

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.