In terms of least complex to most complex:
1. Resto Shaman - very simple mechanics, not much changed from WoTLK, choose between direct heals or chain heal while using Riptide ASAP.
2. Resto Druid - will be a little more complex in Cata with the need to expand from the Rejuv-Rejuv-WG faceroll but still one of the easiest and most forgiving classes to pick up and learn
3. Disc Priest - Won't be able to pure shield spam anymore and has more mechanics to worry about weaving in (reducing WS with heal, etc). Limited AoE healing options
4. Holy Paladin - At 85, pallies will be using their entire healing spellbook on almost every fight. Lots of procs to manage and managing holy power gives a secondary resource system to keep on top of. They are incredibly fun to play at 85, but a lot of people that liked pre-4.0 pally healing despise the new mechanics and the class could be daunting for a newbie healer to learn compared to say a shaman.
5. Holy Priest - Massive toolbox of spells and they absolutely can not get away with spamming any one thing without being terrible (in comparison Discs can get away with shield spam, Druids rejuv spam and Shamans CH spam and still not be completely useless). To be competitive with other healers, you need to use all your tools effectively and the feeling is that you are doing a lot more work to get the same output. Unlike pallies, a lot of the spells have very little synergy with each other, and the entire spec feels scatterbrained. Also, Chakra adds an additional roadblock that you have to worry about and limitation to what you can do that noone else has.
I actually think this is pretty accurate.