I don't think the panel bug (?) is related.
It's rather strange if all this is happening in a stock, proper installation (and not an upgrade from a previous Slack). I cannot reproduce this on my VM. Your user is a member of the right groups. Also KDM, in theory, should take care of stuff regarding user's privileges to mount/shutdown/etc.
Although a long shot, you could try a couple of things:
Switch to init 3, then (as normal user) run xwmconfig and startx.
Create a dummy user and see if it's the same deal:
useradd -m -g users -G audio,cdrom,floppy,lp,plugdev,power,video -s /bin/bash dummy