If not something you've done before or much of, it can be a bit daunting. I don't understand the "gitgud" mentality, because one does not simply "gitgud" over night. And chances are, you will screw up, especially if learning to tank or heal, because it's all a part of learning. I think having the attitude that you want to learn from those mistakes, listen to advice that may help you and improve is a good mentality to have and the ability to joke about mistakes you make too. Case in point, I pulled earlier in a roulette and forgot tank stance, the most simple and most basic and rudimentary thing about tanking, have tank stance on and I've played this game for years and am confident tanking, healing and DPSing. So I just laughed about it to the party I was with.
You will sometimes come across people with poor attitudes to newer or less experienced players, fortunately, they are the minority - a few pages back you'll see my post where I was befuddled being told the healers were terrible when me and a friend were healing...although it was a smooth run and nothing bad happened. So even experienced players get it...I guess there's just no pleasing some people.
Most of the time I find people are positive about people who're trying and have a good attitude, even if they screw up. I think most recognise that people have to start somewhere. And having more people who can fill tank or healer slots means better queues, so why put off people from tanking? Especially if they're willing to improve and be more efficient?