GS, DPS, and being an ******* about it
TL;DR - People keep crying about gears or dps and it ruins the whole party or raid. Each instance has a minimum gear requirement. So stop crying about other people being too low
(... long, multi-paragraph rant that doesn't belongs here removed...)
Also this isn't just a rant, I'm also asking Warmane if you can set up punishments for people who compromise dungeons and raids over such *****ic reasons. I've had way too many dungeons and raids fail because people cry that someone is too low even though they aren't. Seriously, it's just stupid.