I have a feeling that the biggest selling-point features of the expansion are going to be absolutely crap. I'm talking about Expeditions and Warfronts. Expeditions 'might' be fun, especially with the replay value and possible PvP (although it just seems like MoPs scenario, there was already one back then with various possibilities\paths, in Krasarang Wilds, can't remember the name. Collect resources, kill boss), but Warfronts just seems doomed to fail. Just reminded me of a single faction Alterac Valley back in vanilla, people would get resources, and 'dump' them on various locations for different effects, like the huge summon dude. It does not seem appealing at all.
So, in lack of exciting features, i'd say the Allied Races wins as my favorite, specifically being able to play as a Zandalari - upright - Troll. Wooohoo... not enough to get me excited for the expansion though.