Minimum feature set IMO would be (mind that I deal mainly in power, analog and control stuff):
-decent library management with ability of processing multiple components at once
-real-time cross probing
-import/export support for all electronic industry standard file formats + dxf, dwg, step, igs
-interactive routing
-real time DRC
-macro/scripting engine
-multimonitor support
-profile saving/loading for ALL possible settings
-basic 2D drawing functionality (something like 2D autocad, but with actually usable interface)
-smart cursor snapping
-some goodies like via stitching, autofinish track, array paste, alignment tools (top, right, distribute vertically etc)
-schematic snippets
-integrated variant support (hello Cadstar...)