I have two characters at level 90 at the moment. I have done a few heroics without friends. During each heroic, I have managed to make several mistakes.
My best mistakes have been blinking into the boss and starting the fight and using Mind Sear and hitting the boss and starting the fight. The groups I have been in have understood each thing was a mistake.
I honestly thought I would be kicked a lot more than what I have been. So far my count of being kicked is 0. With so many mistakes, I would not blame a group for kicking me along with my very low dps. I am still working on gear and so far each group understands that.
I had gotten in Scarlet Monstery the Graveyard side, almost every single boss drop was a huge upgrade. A random blue staff dropped and purple epic gloves, I had needed on both. Instead of the group being mad about needing on almost everything, they just kept saying Congratulations. That seemed very strange to me though. I am so used to groups grumbling about people needing on bind on equip items that I actually stopped and whispered the warlock on why they all said congrats.
To me, it seems like during the first 4-6months of a new expansion, people are so much nicer in heroics then not. After that time though, people just get down right mean. Like if you don’t have the end game gear or know the dungeon, you are useless.