Hands down the playable content. Trials, Dungeons, and the Raids are an amazing step up from previous expansions, and I hope Square Enix keeps improving on that.
MSQ is something we only do once, but duties is what we engage with every day. That should be the real meat of an expansion.
(Yes, I am aware that on x.0 patches, the only thing one can do is the MSQ, Dungeons, and the Trials.)