OK, so I tried KDE for the last couple of days, and there were some nice things, and some annoying things. I decided to try and track down why the "Gnome Flashback" was not working for me so I could get things back to my old UI and I've got it working again now.
Well... sort of working. I have moved the rather pointless "/usr/lib/gnome-session-failed" out of the way and everything looks good so far.
It's kind of a shame that +GNOME knows something went wrong with the session startup but it's fail dialog won't give me any text to type into Google - it's just a rather futile dialog box that tells me to try again and maybe this time I'll get lucky. Yeah, right.
Of course I look in ~/.xsession-errors and there are a gazillion lines where $LAZY_PROGRAMMER lets me see a "FIXME: assigning images is not implemented" and other riveting stuff, but nothing that obviously suggests what might be making Gnome want to run the "gnome-session-failed --allow-logout". And apart from that annoying dialog covering 85% of the screen (until I rid myself of it) everything seems to be working. Odd.