Trying to set up Samba on a Debian server so that shares can be browsed as a guest, and user-authentication is optional. IIRC this is the default behaviour on Ubuntu.
After hours of messing around, I finally decided to grab the /etc/samba/smb.conf from and ubuntu machine and replace the debian one. And what do you know, it works.

seems the key directive is

usershare allow guests = yes

Ubuntu:

#======================= Global Settings =======================

[global]
 log file = /var/log/samba/log.%m
 passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
 obey pam restrictions = yes
 map to guest = bad user
 encrypt passwords = true
 passwd program = /usr/bin/passwd %u
 passdb backend = tdbsam
 dns proxy = no
 server string = %h server (Samba)
 unix password sync = yes
 unix extensions = no
 workgroup = WORKGROUP
 os level = 20
 syslog = 0
 panic action = /usr/share/samba/panic-action %d
 usershare allow guests = yes
 max log size = 1000
 pam password change = yes

Debian:

#======================= Global Settings =======================

[global]
 log file = /var/log/samba/log.%m
 passwd chat = *Enter\snew\s*\spassword:* %n\n *Retype\snew\s*\spassword:* %n\n *password\supdated\ssuccessfully* .
 obey pam restrictions = yes
 null passwords = yes
 encrypt passwords = yes
 public = yes
 passdb backend = tdbsam
 passwd program = /usr/bin/passwd %u
 unix extensions = no
 dns proxy = no
 server string = %h server
 unix password sync = yes
 workgroup = WORKGROUP
 os level = 20
 security = user
 syslog = 0
 panic action = /usr/share/samba/panic-action %d
 max log size = 1000
 pam password change = yes