Nethaera (Blizzard community representative) has posted some new, interesting information about the leveling experience in Cataclysm. Mount Hyjal and Vashj'ir, the two opening zones of the expansion, will now require level 80 to enter and begin questing in. Betas go through a lot of balance and experience tweaking, and this new change hints at some issues with the previous leveling curve. In addition, creatures' health pools have been increased with the new level restrictions, and level 80 is required to use the dungeon finder for the opening 5-man instances of the expansion, Throne of the Tides and Blackrock Caverns.
Finally, Valnoth reiterated that you cannot enter Cataclysm zones until 80. Raid-geared level 80s will still be able to mow through the masses of creatures, while green-geared level 80s will be given a large ramp-up in power after the opening zones. With only five new levels added on to the game, Blizzard has distanced itself from The Burning Crusade and Wrath of the Lich King models of allowing players two levels under the cap into the new expansion content. Previously, Hellfire Peninsula opened to players at 58, and Borean Tundra or Howling Fjord opened at 68. The decision to close Cataclysm to players under 80 seems like a necessary move to keep content tight in the new zones. Considering that Northrend is itemized already from 78-80, Blizzard can capitalize on the active content to move players into the new spaces.