LFR is designated to be the most easy difficulty to offer an introduction how the late game content looks like,what's obtainable from it and what's the most important part - how it's like to try and work in a team with large group of people to achieve the same goal.
And that's exactly the point where a lot of people stumble. Because, let's be honest,practically every single player joining the LFR queue does it for his own personal goals. Therefore no matter what the goal is on a failure they feel that the other 24 random people are holding him down or bashing him for trying to achieve that. And what you get in the end isn't a team of 25 working together to achieve victory but 25 individuals chasing each their own personal goal/benefit.
Furthermore most complainers usually have gear or experience that exceeds the minimum necessary to join LFR usually even by a huge margin. Therefore their individual performance is a lot higher that the average bunch. It's pretty natural to feel dragged down if you perform together with people that make mistakes on things that you consider common sense. However it isn't. As surprising it may be to hear it's not common sense to evade squiggly purple lines on the floor when Malkorok does his bash. It's common knowledge. And the difference between those is that you actually have to KNOW what's happening to act accordingly.
Which is often forgotten by those who have plenty of experience behind their belt. And here's the part why I think complainers are bad. Because they by no way try to improve the performance of the team. They do not explain the tactics (or where,who and why did wrong) in a sensible way but instead insult other people. They think that the simple fact of them attending the boss fight is all that's required from them. And what I want to say by that is....
If you are not ready to raid in LFR with a group that most likely doesn't know what they are doing and can't be arsed to say more than "STFU Noob Tank/DPS/Healer" and simply don't have the balls to take up leadership of the group and share your experience then keep silent yourself and let other people do their best.
Because honestly if you complain LFR being so much worse than the alternatives then why you just don't use those? That's what they are made for.