From about level 20 onwards you generally find yourself in one or more zones, going back and forth grabbing whatever quests were available and within the right level range. Often this could be quite annoying especially if you have to wait to level before continuing with a quest chain and the constant travelling could really eat into your play time.
Things got worse as you headed towards 60, chances are you'd only be doing 1 or 2 quests in a zone before having to fly/ride off to another zone.
Once you ride through the dark portal things change slightly, suddenly there are entire zones to complete at any one time before moving onto the next one. Each quest hub you approach is suddenly awash with yellow exclamation marks which you can run around and pick up before heading off to complete the objectives. Naturally there will be quests that can be completed at the same time, but you are going to have to figure that out yourself otherwise you may well spend lots of time riding back and forth to the same spot.
Finally the frozen north beckons and you find yourself on the coast of Northrend with a whole new set of zones to quest through and quest hubs springing up. However this time instead of being awash with exclamation points there are only one or two and as you complete quests you either progress the chains or open up new quests to do. The other main advantage is that when doing quests they tend to be all grouped up in the same area.
Of the three models I definitely prefer what has been done in Northrend, the story lines are much easier to follow and I don't feel I'm wasting time travelling around trying to sort out the best order to do all the quests in. I really hope that Blizzard keep this questing model for future expansions, or indeed find some way to improve upon it.