I would argue that the balancing act has got it almost right: in raids, there aren't "best" healers anymore... just different healers.
--Druids have a large number of HoTs that are crucial in fights like Gruul, which have silences, and other fights were healers can be incapacitated; Swiftmend allows them to combat damage spikes at instant speed. They also add a party aura to healing received and an additional battle rez. Survivability is only decent, with no anti-aggro abilities and relatively low armor. Tranquility is a splendid panic button that can be made threat-free through talents and uninterruptable with Barkskin. Their area healing, other than Tranquility, takes the form of just HoTing everyone.
--Shaman have an absolutely terrific raid spell: Chain Heal. It's an AoE healing spell that is independent of parties, seeking out any damaged players in the vicinity. It's a great spell. Single target heals are nothing special, but the utility of their totems is undeniable, with Mana Tide Totem particularly noteworthy. Everyone likes party buffs. Shaman have relatively good survivability with a bit of anti-aggro from talents and a good chunk of armor.
--Paladins have the highest survivability, with the most armor, the infamous bubble, and a passive threat reduction from healing spells. Indeed, there are actually fights in which paladins deliberately build healing aggro for tactical reasons in the confidence that they can take the hit. Paladins have the game's most efficient single-target heals and are notorious for not going out of mana. Paladins have no area heal capability. However, adding a second paladin to a raid adds an entire additional round of paladin buffs to the raid. Triple paladins are frequently desired to give paladins all the buffs a raid wants.
--Priests have medium survivability; they have armor on par with a druid's but possess Fade, a temporary aggro drop, and Psychic Scream for emergencies. Priests have the most versatile array of healing spells, with quick and slow single-target heals, a HoT, a damage shield, and several forms of area healing. They have the unfortunate attribute of requiring talents from two different trees to fill out their healing abilities, while other classes get all of theirs in one; the consequence of that is that priests must give up one of the two talents they'd really like (Divine Spirit or Circle of Healing). This is the only "stacking utility" priests get, but Circle of Healing is merely nice, not overly deserving of a raid spot. Patch 2.3 MIGHT change this by making a deep Discipline build desirable, but we'll wait and see.