New Frontiers was a PvP (and free) expansion. Catacombs is a PvE expansion. Inconnu, Shar, and Kobalds are now getting cities of their own underground and they are adding instanced adventures (like LDON) that can be done solo or grouped. So if you're solo, the instance is automatically tuned to your level range like in LDON as opposed to how it works in WoW where the level of the mobs in the instance are the same no matter what level you are.