Several, actually:
when in Layer View (Preview) mode after slicing, show the physical dimensions (height at least) of the current layer displayed as well when moving through the layers
cylindrical "support blockers" (also modifiable by "per model settings" as the rectangular ones are now).
allow a support blocker modification of "do not print" (with options to enable or disable top/bottom/wall layers for the surrounding printed areas, could just be the children of the base layer settings)
a "compare profiles" option (or plugin) that enables you to compare the differences between two profiles at any time, not just after you've sliced a model and go to change to a different profile, and it should include ALL settings, including the experimental and special modes.
4a) when switching profiles, checkboxes next to the changed settings listed that allow you to choose which settings to keep or discard.
One more thing: The ability to close the window without quitting the program and entailing the entire startup sequence for the program again.