user-defined attributes

Working with other CAD systems I have found the notion of :
TAGS or PROPERTIES that can be added to ANY design element extremely powerful.

Some examples : adding a flag to drill-hole so it will be processed by the export script in a special way
for example ; nibble , laser, or drill-tolerance

Adding a remark/text note to a part on a Component (watch out, keep close to edge, any free text)
Adding a tag to a trace :
'.impedance=50'
'.impedance_tolerance=5'
'.trace_routed=automatic/manual'
'.IPCNET=88912'
'.origin=board.brd,line=777'

Adding tags to a round flash eg : .pad_usage = feducial | tooling_hole | testpoint

Adding tags to a drill hole : .drill_type = plated | non_plated | via

Adding tags to filled polygon, about clearance to board edge, not just other copper, maybe same net?
Add a tag so the item(s) will be excluded from the DRC?

Adding a special flag to a elements in a selection, so you can get back to it later, using a filter based on tag and tag value.
When you share a design file, you can tags for the other person, todo-items on some elements.
The important thing is that you need to be able to set/get the tags/properties inside the scripting too.
You can use the tags to select, mark, and move, copy in a flexible way.
Also tagging an item with : '.look_at_this_BOB' will allow Bob to quickly find what you need him to see.

By introducing some 'standard' tags about 'layer', 'tolerance'' 'library-origin' and 'selections' you can do many things.
You could offer 'temporary' and 'permanent' tags, the first will not be saved, the other would end up in the board file.

My examples are a bit lame now, but I will add some serious use case based on thing I see in other software.