What is the privacy issues with Bcc?
None really. BCC stands for blind carbon copy. When you want to send an email to more than one person, but don't want to reveal everyone's email address - you put them all in the BCC field - separating each person by a comma or semi-colon. Everyone receiving the email will only see their own name - even though it's been sent to more than one person.
BCC is the same as CC, except that the BCC field isn't public. An email sent to you via BCC will look as if it was sent normally and magically ended up in your inbox. This is handy for mailing lists, as you could send an email to yourself or nobody and have all of the recipients in the BCC field, protecting their privacy.