From the doc: Motivations and Goals (more on "a qualitative sense of progression")

The primary motivation behind the design of Micro Missions was to create a game that emphasizes moment-to-moment fun and variety rather than a constant grind to progress and go just one step further. At the same time, the connected nature of the games and the idea of acquiring and exploring a variety of content provide the sense of progress and improvement.

However, this progress and improvement are qualitative rather than quantitative. Instead of filling a level-up or progress bar or gaining points, the player acquires and sees a variety of content, some of which is more suitable for his current goals and is thus an improvement over the old stuff. This qualitative variety and difference should be more interesting for a player to explore than simply adding more numbers to his stats as time goes on.

The connected mini-games structure is thematically cohesive, yet very disparate in terms of gameplay. This design allowed us to create an RPG-like game world that requires a lot less gameplay balancing than an actual RPG, since gameplay rules are not nearly as strictly defined.

The “anything goes” feeling of the game also allows for a game that is more like a crazy, barely controllable bumper car ride than the intentional, directed drive of a typical progression-oriented game that is punctuated by the start-and stop of dying or failure states. Instead of having failure or lose states where the player has to revert to a previous state and try again, this game is more suitable for having diversion  or deflection states, where the action goes on, but goes in a direction that was not necessarily anticipated by the player.