One thing I seemed to notice in Sonic 4 is that the level layout has a whole lot of horizontal/vertical movement. The original Sonic games paid close attention to the shaping and curvature of the ground. It was sculpted to feel natural and create interest and variation even in seemingly mundane locations. Even Sandopolis Zone (a comparable level to the recent Sonic 4 Ep 2 trailer) paid close attention to this.
I feel like this is one aspect that has not been given very much attention in Sonic 4. There are many abrupt vertical walls and when there are slopes, hills, and little dimples in the land, they don't seem to generate nearly as much interest as they did in the classics. Even Splash Hill Zone, while having quite a few slopes and curves, has a huge amount of the level consisting of flat, horizontal land.