There's a lot of people arguing that MoP had way more content than WoD - or the other way around. So let's actually compare both of these expansions and decide for ourselves. I'll compare 5.0 with 6.0 so I won't count anything added in patches.
MoP added 9 new dungeons <-> WoD added 8 new dungeons
MoP added 3 new raids with 16 bosses <-> WoD added 2 new raids with 17 bosses
MoP added 2 new world bosses <-> WoD added 3 new world bosses (you can do 2 per week)
MoP added 8 new group scenarios <-> WoD removed group scenarios
MoP had unique armor sets rewarded from CM dungeons <-> WoD has unique weapons rewarded from CM dungeons
MoP added 53 new mounts <-> WoD added 46 new mounts
MoP added 531 new quests <-> WoD added 401 new quests
MoP added 73 daily quests <-> WoD added 13 daily quests
MoP added 7 new reputations <-> WoD added 8 new reputations (6 of them are faction specific, so you can do 5 reputations if you only play on one faction)
MoP added pet battle system <-> WoD only added a few new unique pets and didn't increase the max level of them
MoP added tillers farm <-> WoD added garrisons
MoP added 2 new arenas and 2 new battlegrounds <-> WoD added 1 new world PvP zone
MoP added 1 new race and 1 new class <-> WoD added new models to 9 playable races
MoP added 8 new zones <-> WoD added 6 new zones
Let me know if I missed something so I'll add it to the list.
So now you can decide which of these expansions had more content.