The phasing in Northrend didn't affect areas in the way it does in Cataclysm, where it is used to phase entire zone areas as you progress through them and where the zones themselves change. In Northrend you would go to a phased area but it would still have mobs, you just wouldn't be able to see quest-specific mobs for one area's phasing. The areas themselves did not really change, as opposed to something like Hyjal where you actually change the zone by progressing through it.