In Cataclysm your stats and health increase dramatically, and the gear, mobs, quests, and experience scale to match that. While in Northrend, each zone gives the same experience (which means you could level at both starting areas instead of continuing to the harder ones), in Cataclysm the different zones give significantly more experience. It is no longer viable to do both starting areas, or stay in a level 79 zone. Don't worry though, the new zones are fast, smooth and fun.
How hard is it to level in Cataclysm?
The experience needed per level increases from 1.5 million at 80, to 9 million at 85. Wow! Don't be scared though, the level 85 zone gives way more experience to match this, and almost all Cataclysm quests are very smooth and immersing. You must be at least level 80 to level in the new zones. Remember to accept the quest for each area at the Hero's Call Board in Stormwind/Orgrimmar to begin the quest chain.
Level 80 - 82
Pick up the quest for Mount Hyjal or Vash'jir at the Hero's Call Board in Stormwind/Orgrimmar. Remember that you should only do one of the starting areas now, because the zones later will give more experience. Also, in Cataclysm you must discover the entrance to dungeons before you can que for them with the dungeon finder tool. However, you can always come back at 85 for fun and gold!
The zones are:
Mount Hyjal and Vash'jir -
Mount Hyjal is the beautiful nature zone located in northern Kalimdor. Vash'jir is the fantastic underwater zone off the side of the Eastern Kingdoms. Remember that every quest line will give you transportation to the correct area.
Blackrock Caverns -
This instance is entered through the Blackrock Mountain. You can get there from Searing Gorge or from Burning Steppes in the Eastern Kingdoms. Enter the mountain, then in the room with the floating circle path, go right. Go through the first entrance (a broken gate). Continue through the path until you see some crates. The portal is there. Picture: (http://3.bp.blogspot.com/_2jOddRkLhw0/TJi5t1VtBpI/AAAAAAAAAzE/rMCUkgy-MEk/s1600/BRC.jpg)
Throne of the Tides -
Throne of the Tides is the one of two dungeons in the Abyssal Maw complex located in Vashj'ir. This dungeon has a pretty creative entrance. You get yourself to the Abyssal Breach and then get sucked down. Where you get dropped in a pool in front of the entrance. Picture: (http://4.bp.blogspot.com/_2jOddRkLhw0/TJjUB-Cy9OI/AAAAAAAAAzU/dwOYh7YuTy8/s1600/throne.jpg)
Level 82 - 83
You've already gotten a full set of new items, and your stats and health are looking pretty nice! You will need these items for the next zone, the mobs are bigger and stronger. If you did both Mount Hyjal and Vash'jir, you will be level 83, and can skip this zone.
The zone is:
An amazing mineral and stone zone located in the Maelstrom in the center of the map.
The Stonecore -
This dungeon is located in the middle level of the Temple of the Earth in Deepholm. Fly about halfway up on the left side of the temple. Picture: (http://4.bp.blogspot.com/_2jOddRkLhw0/TJi63HR9KUI/AAAAAAAAAzM/pTNea0ghsK8/s1600/Stonecore.jpg)