When creating custom reports it would be great to have the ability to create custom groups within the report to aggregate/group property level data. For example, to be able to create an industry report where you're able to group the 150+ industries into say 10 key industries. Right now a report showing 150+ industries isn't very helpful. I realize you can create a new property and use a workflow to assign that property but there are hundreds of variations to reporting based on aggregated data. Another simple use case would be to show deal count within various $$$ values, if you want to see how many deals represent $0 - $10k; $11k - $50k; $51k - $100k; $100k+. With the ability to easily customize the groupings as needed.
While waiting for them to add a native report builder solution, I came up with an alternative using "Calculation" properties. E.g with the "Days to close" property: