Every raid encounter is designed to challenge the players attempting it. Tanks can be faced with controlling several mobs at once, while DPSers often have enrage timers to race against. Blizzard's developers have a few different tricks for putting healers to the test. The first and most obvious danger that healers face is the rate of incoming damage, which we have to counter with spells that are appropriately powerful. We are also often tasked with reacting quickly to stimuli, like instantly dispelling or bringing a raid group's life back to full before another incoming attack lands.
With the release of Cataclysm, the developers specifically focused on a mechanic that had been sidelined for quite some time -- mana management. Our spells' mana costs have been tweaked and mana regeneration capabilities have been altered to ensure that we focus on choosing the right spells. We used to be able to spam Holy Light carelessly in Wrath, but now we can't sustain nonstop Divine Lights for more than a minute or two without emptying our mana bar completely. Learning to live with a limited mana supply is paramount to being a successful holy paladin in Cataclysm.