Cons:
Quest NPC's always dead in a quest hub, for example Thrallmar.
Frustraiting to level up herbalism and mining.
90's flying around killing you, while questing for the lulz.
You see other people from different realms stealing mobs and quest items. Thanks Blizz for a living world. Now go fuck yourself.
I enjoyed being alone. It's not like people chat with these strangers from another realm. No, everyone just minds their business and moves along.
Pros:
There are none.
Blizzard needs to start merging servers together sooner or later.