I always felt healing was the most stressful during progress: DPS/Tanks fuck-up all their movement CD's so you have to save their asses just to see an extra phase and what not. During proper farm it's usually a breeze as you catch up on gear and people know what to do, unless of-course you drop a healer or 2 to make it faster/challenging.
The reason people say healing is the most difficult is because you're not in control of other peoples mistakes, you just have to mop them up. And not playing in a top raiding guild means that raiders fuck-up, a LOT. (The always nice: eats every third bomb and chases the active healer with rapid fire & wonders why the healer is cursing him)
Maybe it's because I was Brain Heal spamming for so long in BC and Wrath but I LOVED my Resto Shaman at the start of Cataclysm. Halfus was probably the most fun I had healing because everyone was low on health the whole time you had to heal the right people at the right time.