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.