pre-tbc, i u wanted to be accepted in a guild 100%, u chose a priest
now its a diff story -_- sadly.
i became shadow after getting bored of healing to the bones
well before thinking about anything else, you have to decide, you wanna deal damage or heal? if you wanna heal, do you want the balanced class priest? the most effecient in straight healing paladins? the heal over time lords druids who need a lot of gold to get respeccing if the guild need an off-tank? or simply u love shamans theri totems and the chain heal?
if you wanna deal damage, do you wanna be ranged hunter/lok/SP/mage? or close range rogue/warrior/druid? (ofc some classes are not preffered as a dps warrior in some guilds) or u simply wanna tank which there are the warrior/paladin/druid but the warrior is first choice, druid second and pala third, paladin is best at aggroing a grp of mobs, really nice in heroics and normal instance ^^
after u decide which kind of class u want, u can THEN think of which class in specific u want according to their advantages and disadvantes ^^