Record issue closing date
I can set this up pretty easily with the custom workflow plugin, but I think it's simple enough that this should be a basic feature...
When an issue's status changes to a state that closes the issue, assign the time stamp to a value in the issue table. Allow the value to be updated whenever the status changes to a closing state in the future (for workflows that include reopening). Like the 'created' date, this would not be a visible field for editing.
My hope is that this extra data will ease the chore of reporting trends and graphs illustrating open vs. closed over time (a frequent request). It'd also simplify building chart plugins that support this sort of analysis.