Just because northrend is going to be a frozen area does not mean that everything will be immune or have resistances to frost. In dun morogh, for example, (dwarf/gnome starting area, which is covered in snow and ice) the mobs are not immune to frost. You can also look at places like winterspring.
Or for another look, most of the mobs in places like burning steppes and searing gorge (excepting fire elementals, which are all immune to fire) most of the mobs there can be hit by fire spells.