If I can't choose EVERYTHING!!!! as my choice I have to go with the story, the fact that it's being leaked out so slowly in the flavour text and card images is awesome.
The gameplay gets me to come back for more every time, but the storyline is what makes me want to speculate wildly about it 24/7
I'm a sucker for metaplot--RPGs like Deadlands and Vampire, with tons of backstory and setting material really interest me, particularly when that metaplot advances, based on in game actions. As you can imagine, I fell in love with SotM's unique world pretty quickly. I'm invested, at this point--I want to see what happens with Legacy, which Visionary triumphs, and how the Final Wasteland can be averted.
I love the fact that a round of Sentinels plays so cinematically. The game reminds me of nothing so much as the last third of The Avengers film, where the camera shifts from hero to hero as each gets their chance to be more badass than the last. A round of SotM makes me visualize Tempest charging up a ball of lightning, then blasting an alien ship, forcing a new deployment. Ra waves his staff, and a cascade of fire immolates some of the troops before they hit the ground, while a tendril of fire licks across Expatriette's submachine gun as she fires into a wave of gene-bound goons.
i love the fact that no two heroes play the same, despite using the same fundamental mechanic. Legacy and Argent Adept are both "buff bots", but play nothing like one another. Haka and Fixer are both melee bashers, but are totally different in feel and play style. SotM achieves that zen balance of complexity and simplicity.
And really? I love how excited my gamer friends get about the game. We're relatively new to SotM--I picked up the base box as a self-Xmas present, the fist two expansions soon after, and received everything else just yesterday. I told my gamer buddy Chris about The Scholar and Chrono Ranger, and he's already dying for another go-round. It's a joy to have at our table, and that's all you can ask for.
All the time for me too, man. All the time.