random duo-ing thoughts:

2dps classes make quest leveling nice and quick (hunter/hunter for instance)

healer+tank makes finding three DPS for instances pretty easy (druid/priest/pal + druid/pal/war)

healer+dps gives you some flexibility when allout DPS just doesn't quite cut it. (mage+priest is one of my favs for 2boxing), druid+rogue is also a lot of fun.