It's not the hand-animated visuals, crafted by artists whose resumes include My Little Pony: Friendship is Magic and Atomic Betty. It's not the challenge of matching each dragon with their favorite treasures, collecting and combining items to create more potent pieces.
No, it's definitely the vomiting, followed closely by the rainbow pooping.
East Side Games' Dragon Up is not a Dragonvale-type breeding game as much as it's Tiny Tower with collectible dragons instead of people. You purchase eggs, they appear at the top of your aerie. Villagers and wizards regular come to pay tribute to your magnificent barfing beasts, carrying their gifts to their intended recipient via a boat that's an awful lot like the slow-moving elevators that climb those minuscule skyscrapers. It's that same simple, addictive formula that will have you hesitant to turn the game off because you know another gift is seconds away from arriving.
Money is generated by sending hatched dragons with silly names on quests. They disappear in a rainbow flash and reappear moments later. Then they spend hours horking up gold pieces. It's beautiful.
The higher the dragon's level, raised via the treats villagers bestow, the more lengthy and profitable the mission—and the more they puke. It sounds disgusting, but it's really charming in a twisted sort of way.
It certainly helps that these dragons aren't your normal garden variety magical lizards. Jelly dragons, commando dragons, fruit dragons—my personal favorite is the 8-bit dragon, unlocked via an introductory in-game purchase of $1.99, along with some meter-reducing gems.
One can even form or join a guild, should they wish, but the benefits of doing so escape me as no one has accepted my invitation to the Kotaku guild yet. *weeps*
Dragon Up sets itself apart from the dozens of other dragon breeding games on iOS with its Cartoon Network style, its silly sensibilities and the fact that, as stated previously, it's not really a dragon breeding game. It's a dragon vomit harvesting game, and that makes all the difference.