FreeKaner
Prophet of the Dumpsterfire
The top guilds are probably anal about it, but I wasn't in the top guild yet when they told me it doesn't really matter and I should go anyway. I had just dinged 60 and had gone once with the guild master and some other peeps to Stratholme. We killed everything in MC without breaking a sweat. This was very early in the server's lifetime, probably 2 to 3 months max after it launched. As for unpopular specs, it depends on the amount of memeness, balance druids and prot paladins are a no-no, but some guilds take an enh shaman with Nightfall (as opposed to a warrior with one) or a shadow priest (as opposed to a Holy/Shadow hybrid that can keep up the debuff while being more useful) for example. And even then, if your guild master is playing a druid, like ours was, he can force his way into being balance for some raids that are on farm.
People who are good at the game are more willing to raid with suboptimal gear than ones that are bad. This has always been that way. Guilds that were good are willing to try at first raids both in vanilla and later in expansions with just questing items while worse guilds wanted everyone in pre-raid BiS and PUGs want everyone to already outgear the content.
It's a myth that more casual guilds allow worse gear. PUGs are alwasy going to be most demanding gear wise.