When I compare activities, particularly identical courses, you can only do so with summary data (e.g. avg HR), I would like to be able to see the selected activities plotted on the graphs that you see in the activity detail screen. This would be an excellent, graphical, means of seeing performance improvement, when the time and/or distance is not a variable (e.g. indoor rowing).
Failing that, a means of exporting the detail data as a CSV file, for those of us who have not the time/technical ability to mess around with xml schemas.