it would be nice, in case of field with e.g. labels, to say, what I want to do with it in bulk operation, to Append or Remove values.
when I select bunch of issues based on custom criteria, all those issues could have labels already, and I do not want to lose them. when I would check Append, labels I chose would be appended to selected issues, when I would check Remove, only those I chose would be removed.
to me multiple value fields are: labels, fixed versions, affected versions, watchers, components.