Healing and Tanking is way more difficult than DPS in my opinion and this is why;

As a Tank; we have to deal with people who think they know their job, to be fair, there a lot of decent players however that credibility goes out the window when they are poorly geared, can avoid aoes ( or understand that it's better to avoid then let action / spell trigger ), or follow simple mechanics thinking Healers and Tanks are to blame for short comings. Also healers who lack common sense of not healing while pulling or adding regen.

As a Healer; Same above, just add tanks that don't use their CD or stick around to get hit, can't face mobs properly, or think too much of themselves not using tank stance giving healers a hard time.

Over all DPS can be power houses and a Decent DPS will be able to keep tanks on their toes. ( Extremely rare as of resent )