Showing the colored buttons, which you can't see in 2.1.3 but will if the MIDI playback project succeeds.
Not mentioned is the "stretch" editing operation, which you can do in 2.1.3, but it behaves badly.
I have said that if we invite more use of MIDI tracks, we must either fix this feature or disable it. I am offering to fix it, to let Pokechu22 concentrate on getting playback to work. The parts of code to change for this and that don't overlap.