I've got a bad feeling that whinners and people who don't understand how to play are common problem. So, how can you tell who these people are before grouping with them?
Thankfully once you /ignore someone you won't be put into the same group by the LFG tool.
A lot of players in my guild share if someone really messes up so you can already make sure to get around the bad experience of grouping up with them without having to meet them first.
This might sound harsh, but keep in mind I'm not talking about turning up in insignificant gear, accidentially pulling or not knowing the instance.
Leavers, slackers, flamers, people who roll need on absolutely everything or use no more than two skills (yes, had someone like that for NMP just a few days ago O.o) ... watching every single cutscene in Black Talon ... those are things where I simply don't want you and me in the same group.