I do think the main standout aspect of Candy Crush is that it has different success criteria per level (quotas vs. ingredients vs. clearing jelly.)

It also has several different scoring mechanisms: each level has a score, each level has a star rating, each set of levels is a score, you are scored vs friends, and scored vs strangers. It even has some kind of theme for those that like that (I always skip the cut screens). So it appeals to different people who enjoy different kinds of challenges.

It has scarcity: some levels are blocked off temporarily, and some game elements don’t appear until later, so you want to get to them more.

It has frequent “power” boosts through combining special candies, and the satisfaction when you can combine rare combos and get lots of action.

It is unexpectedly strategic, requiring more planning than a typical game of shoot’em which only requires fast reaction skills. And thus rewards a little thinking.

It is viscerally enjoyable: the colors and candy shapes are attractive, the explosions and motions of the candies connote a satisfying accomplishment (more than simply disappearing from the screen, for example).

It provides many unexpected bonuses: studies indicate that unexpected bonuses, or those awarded at unexpected times, are worth more than “earned” bonuses.

It is free and widely available.

Yehuda