Is there some permissions setup that would allow a user, say, john, to add a file to a directory d, but not be able to remove an existing file owned by another user, say, root? My understanding is that this is not possible, since one needs execute permissions to add a file to the directory, but this also gives on the power to unlink any file in the directory.(I'm using Mac OS 10.9, but this question presumably applies to all POSIX-ish systems.)
Ability to add files to a directory but not remove existing files
files;permissions;directory
Yes, to do so, you would want to set the sticky bit for that directory.excerptAnother important enhancement involves the use of the sticky bit on  directories. A directory with the sticky bit set means that only the  file owner and the superuser may remove files from that directory.  Other users are denied the right to remove files regardless of the  directory permissions. Unlike with file sticky bits, the sticky bit on  directories remains there until the directory owner or superuser  explicitly removes the directory or changes the permissions.That is, you would give user john execute permissions on the directory d so that they could add files to it, and then mark the directory as sticky with chmod +t /path/to/d to ensure that john (and any other users with +x permissions) are only able delete files (or subdirectories) that they own.