How can I debug an issue with Xubuntu 12.04 (fresh install) which just waits idle after a login for about 30 seconds?

The login screen is delayed correctly. After login, I get my desktop background, but no panels or auto-starting apps.

It doesn't seem to be an authentication/pam issue, because I can login without delay at the console while the graphical session is still stuck. There's no disk or cpu activity and no obvious respawning of any process when I look at htop. There's nothing obviously wrong in .xsession-errors. Most interesting errors:

openConnection: connect: No such file or directory
cannot connect to brltty at :0
WARNING: gnome-keyring:: couldn't connect to: /tmp/keyring-wFn4VR/pkcs11: No such file or directory
...
(polkit-gnome-authentication-agent-1:2131): polkit-gnome-1-WARNING **: Failed to
 register client: GDBus.Error:org.freedesktop.DBus.Error.ServiceUnknown: The nam
e org.gnome.SessionManager was not provided by any .service files
** Message: applet now removed from the notification area
** Message: using fallback from indicator to GtkStatusIcon
...
(xfce4-indicator-plugin:2176): libindicator-WARNING **: IndicatorObject class does not have an accessible description.
...
(xfce4-indicator-plugin:2176): Indicator-Application-WARNING **: Unable to get application list: Operation was cancelled

Bootchart seems to end before I login, so it's not that helpful.

Where else can I look for information?