(1) Retrigger parameter on style tracks (Rolands lack this, all other brands have it)
(2) User-adjustable recognition delay, to prevent the premature sounding of unintended chords. (LiveStyler has this, hardware arrangers don't.) It makes a huge difference if you play with simplified fingering. Kudos to Korg, which have a pretty forgiving fixed "window" programmed in by the factory. However it should be adjustable from real time to about 60 microseconds, and all brands should have this.
(3) Simultaneous use of and interaction between Song mode and Style mode. Yamaha has elements of this, in a limited way. Korg has two sequencers and they don't talk. Boo!