I currently love and use the "Compare Time Periods" option frequently, but it only works in certain situations (for instance, it isn't an option if I have "days" as a dimension in the report), and I can't drag in Time Periods I've already created. I frequently want that sort of "compare" functionality on all sorts of different columns.
What I would love is to always be able to create a column that compares two other columns in the table. Summary change is nice but not if you have a lot of items in the report to compare. Quick calculated metrics and such are nice but don't give you flexibility with time periods.