It’s interesting to see graphically simple challenging games like Flappy Bird, Geometry Dash, and Phases. All of which have game play that challenges the player, and it is a point of renown for those that achieve the highest scores.
This trend seamed to start with Flappy Bird, which has a simple touch control that controls the bouncing motion of the bird. The challenge for the player is in timing the motion to find the games through the pillars. The graphics hearken back to Mario, so much so that comments about a law suit appeared on the Internet. The game topped the Apple iStore, spawned many clones, and the creator pulled it down for a little while.
Piano Keys has simple black and white squares that the players much touch in rapid succession and keep up with the cascade of tiles that flow down the screen. The challenge lies in the number of piano tile that you can touch within a time limit, the speed the tiles move, or the how long you can last.
Geometry Dash is another game that exemplifies simple game play with stylised graphics. The player needs to move a cube through a series of side scrolling platform environments (levels) with various spikes, walls, and pits by jumping the cube over or under these. Alternative parts of each level have the cube transform into a spaceship that flies through a similar level. The spaceship naturally falls, and touches cause it to fly up at a sharp angle.
Phases has simple graphics, with the player controlling the left and right movement of a ball and it bounces through an dangerous spiky environment.

All these games share similar traits from the arcade games of the ’80s, in that they have simple stylised graphics that just get the idea across, and the focus on challenging game play with the aim of making the game superior.