Although it does not include all the patch notes, I thought I'd add this now so you get the general idea of what will change in the next major World of Warcraft Patch 2.4.0.

*Update* more patch notes added here. 

Patch 2.4.0 will be the second patch of 2008, and most likely the last one before the release of Wrath of the Lich King. The main feature is obviously the new Sunwell Isle and the two new dungeons, Magister's Terrace (5-man) and Sunwell Plateau (25-man).

PTR Release - Probably February, could be the end of January if Blizzard is fast enough.
Live Release - Add 2 or 3 months to the date of PTR release.

Sunwell Isle
Sunwell Isle will be a new zone located north of Silvermoon and will feature a major questing hub heavily linked to the new 5-man and 25-man instances : the Magister's Terrace and Sunwell Plateau.

Quests

• A lot of daily quests will be available (daily quest limit per day will be increased from 10 to 25). Most of them will be here to let players help building and reinforcing the expedition camp built by the new Shattered Sun Offensive (SSO) faction, improving it over time. (Based on the same idea as the Ahn'Qiraj war effort)
• Reaching a high (most likely honored) level of reputation with the SSO will unlock the 5-man instance.
• Having enough daily quests completed on the server will let players access the last 3 bosses of The Sunwell Plateau.
• We'll finally see the end of the Tempest Keep story arc. This will also close most of the Burning Crusade storyline before the release of the next expansion, however there are a few scenarios left to provide players with more content if the expansion gets delayed.
• More parts of the lore will be available to non-raiding players, Blizzard's philosophy is now to let as many players as possible experience it. (This is why Kael'thas has been re-added as a 5-man boss in SW5)

• This is a 5-man instance, with both normal and heroic modes available
• You will probably have to be Honored with the SSO to be able to enter it.
• The design of the dungeon is mostly drawn from Silvermoon City, to let alliance players have a look at the Blood Elf architecture.
• Players might be able to access a Blood Elve Observatory to directly look at the Sunwell.
• Kael'thas is confirmed to be a boss of the instance, and will probably only appear in Heroic Mode. He's still alive and is now powered by a huge green crystal etched to his chest.

• The Sunwell Plateau will include indoor and outdoor environments, with the whole dungeon powered by the energy provided by enslaved Naarus.
• You will encounter 6 bosses in the instance
  • Brutalicus, a giant pit lod who has lost his wings.
  • Kalecgos, a blue dragon enslaved by Kil'jaeden. (Kalecgos on WoWWiki)
  • A Fel Dragon
  • Female Eredar Twins
  • A Dark Naaru
  • Kil'jaeden (Final Boss) (Kil'jaeden on WoWWiki)
• The last 3 bosses will be unlocked once enough daily quests have been completed for the SSO on Sunwell Isle. (Server-wide)
• Bosses will be slightly higher than the ones encountered in Hyjal Summit and the Black Temple, the instance is tuned for Tier 6 geared players.
• Loots quality will be above Tier 6
• There won't be any Tier 7 set, however some sets will be available (but not class specific). For example, you'll find a warrior tuned plate set, but also another one tuned for paladins.

• There isn't any details about this but according to this post, the leveling of gathering professions will be made easier in 2.4. The increased amount of raw materials available will most likely result in a lower cost to level other professions like Blacksmithing or Engineering.
  
• Gaps in cooking leveling will be fixed as well.

• Some changes will be made to classes to improve the balance in PvP combat.
• The changes to Shaman's Earth Shield will probably be reintroduced in this patch, a 30 seconds will be added and its mana usage most likely halved.

• The combat log mechanics will be revamped. This will probably result in much more accurate damage and threat meters.