Most players choose their role based on what they want to do the most -- if you like playing as a ranged character, Hunter is probably a good choice, while being a Rogue will let you sneak around. But if playing in groups is your thing, says GoW, then you should aim for one of those roles that groups want: healer or tank. That's one reason I personally chose a resto Shaman -- everybody always needs a good healer and/or tank, and with a Shaman, I can fill that role, or go off and do my own thing as well.
GoW compares it to being a woman at an engineering school, where you've got your pick of the 1:6 girl/guy ratio (my alma mater, fortunately, had odds stacked slightly the other way). But other than the hot engineering sex, the point is a good one: if you're on a realm suffering from a lack of tanks and healers and you want to group up, why not fix things yourself and roll up one or the other?
[Via Mystic Chicanery]