This comment by Enkylanos on Azure Shadows’ We Didn’t Start the Fire post sums up the added difficulty of tanking / healing for me.

It’s *often* the case that a single mistake by a healer or tank can wipe the raid. It’s *rarely* the case that a single mistake by a DPS can wipe a raid (though the DPS may die, or his or her damage done may be poor that fight).

I know that when I play DPS in groups I make stupid mistakes but there is that margin for error.  I can over-aggro, blink into an un-aggroed mob, knock my keyboard onto the floor and most likely the group will be just fine.  My toon may be on the ground dead whilst I die of embarrassment but nine times out of ten I get rezzed and the run goes on.  If I forget to hit the right cooldown  or stand in the wrong place or let my attention wander a little, the result is often a refreshing run back from the nearest graveyard for everyone.

To make things complicated, it’s also harder to objectively measure your performance as a tank / healer.  As DPS you get a nice simple number and as long as your DPS keeps going up and you’re not pulling aggro you’re doing fine.  As a tank or healer I’m always wondering if I’m doing a good job, or if the tank/healer/DPS is carrying me.  There isn’t a nice simple number to look at – without a deep theorycrafting analysis the best you can say is “Nobody died, so I think I did okay.”