I think the biggest draw of Fire Emblem: Heroes is the fact that any character you can remember from the franchise can be summoned, and used to do your bidding. The roster is so diverse, and with skill inheritance, you never know who might finish you off in the arena. Probably the whales who have thrown the most money into Orbs.
Gameplay aside, the artwork is really amazing. It's way better than what we got on the 3DS titles. You can actually tell which characters the icons are representing, instead of those tiny blurry 3D models! No longer shall we squint with confusion when attempting to move around units! I would even go so far to say that they put too much effort into this game for it to be just a pay to win mobile app. I really hope all this artistic effort goes into future Fire Emblem console titles.
Spoiler: Cordelia is the best character.
Fire Emblem Heroes - Build List 
