If you run as sudo (IE launch terminal and command 'sudo nautilus') do you get thumbnails? It works for me when when ran as sudo, but only as sudo. I have installed webupd8's Unity patched Nemo, but it has the same symptoms (preview of media files fail unless ran as sudo).

I resolved this (rather worked around this bug) by running 'sudo chmod -R 777 /home/x/.cache/thumbnails/'. 777 is required (tried all other combos that make sense, but only 777 works). You will likely have to set up a script to run that occasionally...

Before resolving this issue please check to see if gnome bug 711220 is relevant! https://bugzilla.gnome.org/process_bug.cgi

IE if you open your pictures folder (or other directory with lots of previewable items), then run the command 'top', is nautilus using high cpu? If so and you navigate to a different folder (without previewable items) does that relieve nautilus's cpu utilization?

If it does PLEASE, PLEASE, PLEASE, confirm that bug!