Path of the Titans is gone
The whole Path of the Titans idea has been removed from the game. It will no longer rely on Archeology or on any kind of PvE/PvP progression and instead Blizzard will just introduce a new type of glyphs: Medium Glyphs. Medium Glyphs will add a "fun factor" to abilities, and it wouldn't be surprising to see something similar to the Fortification or Breakthrough glyphs first previewed during Blizzcon 2009.

Guild Talents are gone
The guild talents are gone and your guild now gets extra bonuses depending on its level. There are 25 levels and each level will automatically reward with more cool stuff. The leveling process remains unchanged and your guild will gain experience through PvP, Dungeon and Raid progression, questing, etc. Guild currency has also been removed and rewards will just be "unlocked" after you reach a specific level or complete a guild achievement. Once a reward is unlocked, members will be able to purchase it with gold. Some of the rewards include guild tabards, mounts, heirlooms, and it looks like you will finally be able to have a guild tabard on your mount. (Just like the Argent Tournament banners) New members of a guild won't be able to buy everything directly, they will have to contribute to the progression of the guild before they can access the top rewards. Each time a player helps towards the leveling he will gain reputation with the guild, the best rewards will require players to be exalted with their guild before they can buy it.

Archeology will be less important
With the Path of the Titans gone, Archeology will become a "true" secondary profession and you won't really have to level it anymore. However it will still be used to let you get cosmetic/optional items, get more information on the game's lore, and ultimately it looks like we can expect rewards similar to fishing. (Nothing mandatory, but you'll be happy when you actually find something)

Raids & Dungeons Split
Raid leaders will be able to split an ongoing 25-man raid into up to three 10-man raids. Those 10-man raids will be able to continue from that point on, with bosses already down. So far it looks like it won't be possible to merge 10-man raids into 25-man raids, for obvious reasons. Raid IDs should be much more flexible. You will be able to join any raid as long as it doesn't have any bosses up that you have already killed. There will be more Algalon-style encounters — ultra-hard optional bosses for hardcore players only.

Rated Battlegrounds
Rated battlegrounds will be available for pre-mades of 10, 15 and 25-player sizes. The amount of Conquest points rewarded will be capped so that players who enjoy both arenas and rated battlegrounds don't feel they have to prioritize one over the other. The classic Honor titles will be coming back and rewarded to the top teams each season. The "Gladiator" equivalent teams will also receive epic ground mounts because they want to reward something you can ride in the battleground and be proud of / show off.

Grim Batol Raid
The Grim Batol raid instance will be called the Bastion of Twilight, and you get to fight Cho'gall as the end boss! If you manage to defeat him in Heroic Mode, an "horrific secret will be revealed when he dies". This is an entry-level raid, just like Naxxramas in WotLK. It will feature 5 bosses, and an Algalon-style optional and ultra-hard boss.

Skywall Raid
As expected, Al'akir will be the end boss of the Skywall raid instance. Bosses will include a djinn, a storm dragon, and a lesser elemental lord. You will be able to use regular flying mounts to fly from platform to platform.

Deepholm
Princess Therazane will be a questgiver in Deepholm, in a similar experience to the Sons of Hodir's questline in Storm Peaks. The Ogrim's Hammer has crashed in the zone and the Skybreaker is still in the air. One of the questline will require you to find out what happened.

Heroic Deadmines / Shadowfang Keep
Heroic versions of classic instances might not be ready for the release of Cataclysm and will be shipped in content patch instead.

Old world changes
Stormwind has been completely redesigned to accomodate flying mounts. The Park of Stormwind has been destroyed by Deathwing and the Dwarven District now hosts a Bank and an Auction House.
Orgrimmar has been redesigned as well: Garrosh is the new Warchief and is located in the center of Orgrimmar, not in the Valley of Wisdom. Goblins have their own area, the Valley of Wisdom becomes a Tauren district, and there is now a back gate leading directly into Azshara.

Miscellaneous Changes/Information
Wrath of the Lich King had 1000 Quests, Cataclysm has over 3000. The current heirlooms will stop growing after level 80. There will be a whole new crop of heirlooms for Cataclysm. Goblins and Worgen will be able to start using heirlooms immediately at launch.