For me, it was scenarios. Pugs don't hold much allure for me. If it's not either a quiet evening on my own or a night with my guild, I'd just as soon do something else. But scenarios ... Scenarios have me hooked. I like that the groups are smaller than pugs. The format feels less mechanical. I guess we'll all eventually fall back into a fair share of silent but deadly scenario groups, too, but for now they feel like a collaborative effort worth biting into.
What from Mists of Pandaria has snuck up and ganked you with a bomb of amazement and delight? Is it a feature, a zone, an NPC, the monk class, the pandaren ...? What are you enjoying now that you didn't anticipate liking quite so much?