After playing this game for quite a long time, I have seen the community as a whole decline quite a bit. This is quite a bit of a rant, so please do not take offense to anything that is said here that may pertain to you.
There is nothing wrong with being a casual. You have a life, you have kids, too much work, other things you want to do. That is absolutely fine. What is not okay is just being bad for the sake of being bad. There is a handful of players who just have no intention of bettering themselves as players. A lot of this is due to SE's hand holding of players. Telling people good job for every small thing they do instead of actually encouraging them to do the best they can do.
My issue with the general attitude from a lot of the casual or "hardcore" community stems from a lot of the entitlement they think they have. Tanks not doing their jobs, Healers not healing, DPS being abysmally low on charts. Now it could be that they do not know they are not performing well (Add an ingame parser SE) as someone is telling them they are fine. Then they have an encounter with someone who is not satisfied with what they are doing and they get upset.
When someone gives criticism, whether good or bad, it is because they feel that you are doing something that they believe is wrong. I am not saying that all criticism is correct, but I do believe you should analyze all that is directed towards you.
If someone tells you your damage is low, don't get butt hurt. Go try out SSS and improve your rotations on dummies.
If someone tells you that you are a bad healer, it might not be because you are not healing, but because you are not doing everything you can do. Healing is one of the hardest and most versatile roles in the game. You control a lot of what goes on in fights. Don't be afraid to experiment, by DPSing when you know a tank is fine, using cooldowns like divine seal and soils outside of boss fights.
For tanks, simple things like holding aggro and positioning is almost all everyone is looking for. If you are going to pull big packs of mobs, use cooldowns to help the healers out.
TL;DR: Don't respond to criticism in a negative manner, instead evaluate it and use it to improve yourself.