My squad raids with:
Bear tank, boomer OS
Arms warrior, prot OS for 2 tank fights
Holy pala, with retri for 2 heal fights
Resto druid
Disc priest, shadow lolspec
Boomkin (me)
Mage
2 Rogues atm (we used to have a warlock before the second rogue joined)
Elemental shaman
Our group has never reached a boss which gave us difficulties due to our raid comp. We emphasize more on bringing people that know their stuff, and like to joke around on mumble during farm (srs bsns can gtfo). We have had heroic DS on farm for 5 weeks i believe, and had very steady progression after our christmas break and till we killed dumbwing.
Conclusion:
Don't bring a specific class, or someone who has more purples than the rest. Bring the player you know will do well, and fits well within the group synergy