The portals to main cities were removed because they were used for previous expansions. They were there to help you reach class trainers and the auction house, which weren't present in Dalaran or Shattrath at the time.
One portal for each faction was added back i, which will take you to either Stormwind or Orgrimmar, depending on your faction. These were added to assist players that have been away for a long time and did not know how to get back to the main cities.
The other portals are not coming back, as in Cataclysm you are now able to fly in Kalimdor and Eastern Kingdoms, and Blizzard does not want players to have their hearthstone in an old city from an expansion they've outgrown.
However the portals to Wintergrasp and the Caverns of Time remain.