Here's my thoughts on the matter:
Everyone in a group has a role they are expected to, at minimum, perform. For tanks, this role is to keep aggro on enemies, and stay alive as long as possible on their own. For DPS, this role is to simply deal as much damage as possible as fast as possible for extended periods of time. For a healer, it's to keep everyone in the group alive outside of each individual's own self-sustain.
If you do this basic job you are expected to do, you are meeting expectations set forth by the role system. However, just like in real life, people don't just want your bare minimum.
Imagine you were hired to clean a room, and despite the room being very untidy, it was really only specified that "clean" meant to "remove all trash" from the room. Sure, you can remove all of the trash from the room, but will your employer take you back just because you TECHNICALLY met expectations? Possibly, but they are more likely to put faith into someone who will do more than their bare minimum. Someone who will tidy the shelves, straighten the paintings, pick up toppled chairs - someone who will, not necessarily go incredibly above and beyond, but who will at least do the bare minimum and then some.
These people who do more than the bare minimum are:
The tanks who, instead of spamming emnity skills to get as much threat as possible and never swapping into dps stance or using dps combos, actually utilizes their entire toolkit to remain JUST above the rest of the party's emnity so that they can hold aggro while killing the mobs faster.
The DPS who, instead of standing still and doing their rotation to pump out maximum dps, are careful - they avoid taking unnecessary damage and wisely utilize their defensive capabilities and other tools to reduce the load on their allies, while still dealing good dps and getting the enemies down fast.
The Healer who, instead of standing around picking their nose when they don't have to heal, stance dances and assists their fellows in taking down the enemies which are causing damage to their allies as well as healing, mitigating, and otherwise preventing the demise of the group when they need to.
You can technically get by with the bare minimum, but it won't be as efficient and, just like a bare-minimum employee in the real world, people may opt to stop bringing you along if they find someone willing to put in more effort into their craft than you are.