I've just recently gone back to LMDE from Gentoo (which I love, but too easy to break) as I really like the Debian editions and what the folks here have done with it. Before I left I was using the XFCE edition and in Thunar or Nautilus I could just do smb://SERVERNAME/Share and it would give me the user/pass window. Tap it in and voila.
Now, it never makes the authentication and just repeatedly asks for me to try and login and then fails.
From the command line, I can mount to a share without issue so it's definitely a GUI based problem.
Any idea on missing packages? I've got smbclient & gvfs-fuse installed.