Not having constant fear/seduce and snare protection is a real game breaker for me when boxing healers. Everyone knows to fear bomb boxers. They will do it over and over until you are out of immunities. With shaman they will run out of fears and/or you will become immune long before you run out of totems - you get 4x a minute, 8x every 3 minutes with the talent that resets cooldowns. It is a raid wide fear break as well. The same with windwalk totem, 4x per minute, 4x more every 3 minutes. It's really really hard to get snared with that, it breaks snare/root and then gives you 6 sec immunity.

I can see multiple paladins and/or druids being extremely effective as boxed healers, and some of their abilities are better than shaman's as far as healing... but invariably you will be fear bombed and caught with your pants down.