From Ghostcrawler

Cataclysm Dungeons difficulty
The normal dungeons require a little more crowd control than many LK players are typically used to, but they aren't intended to be brutal. You shouldn't need to farm a lot of gear before stepping into one. Keep in mind that feedback is going to be all over the place in a beta. Some players are heading into dungeons they know nothing about and getting frustrated when someone can't immediately explain the boss mechanics to them. Some just have no sense in investment since this isn't their "real" character (such irony). Some of the bosses are overtuned and some of the class abilities are undertuned at this stage. Because Uldum and Twilight don't have quest rewards yet, there are also some folks who are just really undergeared. Half your glyphs probably don't work. The new enchants aren't readily available. And so it goes. That sort of thing will all get ironed out over the next several weeks.

You shouldn't have huge mana problems in a normal dungeon. Your middle heal should get you through most of it just fine. You'll have to use more of your emergency heals in the heroics and raids, and you'll also be expected to gear up a little more for the heroics and raids. In Burning Crusade, players went and finished the Shadowmoon quest lines or got their reps to Exalted or maxed out their trade skills in order to have the best gear available for the heroics and raids. That is our model for Cataclysm.