Some packages add new user accounts / groups on emerge. I think it would be nice if they removed these on unmerge. That way, the system would really become as clean as it had been before the emerge.
This deletion would include the accounts' homes and all superordinate directories created solely for those home dirs. E.g., lighttpd creates the user lighttpd with home=/var/www/localhost/htdocs. On unmerge, not just /var/www/localhost/htdocs should be removed, but the whole of
1. /var, if empty (except for the subdirs www/localhost/htdocs); if non-empty, then
2. /var/www, if empty (except for the subdirs localhost/htdocs); if non-empty, then
3. /var/www/localhost, if empty (except for the subdir htdocs); if non-empty, then
4. /var/www/localhost/htdocs, if empty.
This functionality should be taken care of easily by adding the homedir and its superordinates to the list of package-owned files (don't know if this happens purposefully now).
This would also take care of the SNAFU of a package unmerge deleting its own user's homedir. Take again lighttpd, which creates the account lighttpd with home=/var/www/localhost/htdocs, but also claims this dir as its own. On unmerge, it deletes this dir, if empty, but the account remains - homeless.
The account-removal feature could be a FEATURE then.
*** This bug has been marked as a duplicate of bug 53269 ***