The latest build of Runner2 adds way more background detail, animation, and interactivity than I saw at GDC; there are now particles everywhere, flashy effects when you pick up a powerup item, and detailed, animated ... stuff going on behind CommanderVideo (but not too detailed for it to affect clarity.) There are fun elements like an airplane whose wings you dash across in one level, and a new cannon activity that lets you shoot CV at a target at the end of a stage if you pick anything up.
The most eyecatching new element added since my last playtest is "Unkle Dill," a man-sized, anthropomorphic pickle who hangs out at checkpoints. He runs and dives at you in futility if you decide to skip the checkpoint, basically providing incentive to skip every single checkpoint.
Gaijin Games posted some footage of the PAX East build, allowing you to see Unkle Dill for yourself.