This post is mostly for my own reference but if Google sends anybody here, hopefully it'll help them too.
tl;dr version: it seems that the gstreamer-ffmpeg package is required for Thunar (or tumblerd, specifically) to render thumbnails of video files on the XFCE desktop for Fedora 16. This package is provided by RPM Fusion yum repository (rpmfusion-free-updates specifically).
It seems to be pretty hard to find this information online. None of my Fedora XFCE systems were able to render thumbnails of videos, and this caused me some headaches with removable drives (when I'd be done with the drive and want to unmount it, I wouldn't be able to because "the device is still busy"... and lsof /media/Cyro would show that tumblerd was the culprit... it was trying to render thumbnails of any video files I may have seen while browsing in Thunar and wasn't having much luck with it).
The usual set of packages that provide all the video codecs for me didn't do the job (vlc, smplayer, gstreamer-plugins-{good,bad,ugly}), I even tried installing ffmpegthumbnailer and no luck. Finally I installed gstreamer-ffmpeg, and after doing a killall thumblerd and then visiting a folder full of videos in Thunar, it finally worked and started making thumbnails.
Ever since I jumped ship from Gnome to Xfce a few years ago when Gnome 3/Shell was first announced, I've configured my Xfce desktops to strongly resemble the standard Gnome 2 panel layout.

Click the screenshot for the full size version.
Along the top panel I have my Applications and Places menus, app launchers for my commonly used programs, my CPU usage graph, notification area and clock. On the bottom panel are my task bar and workspace switcher. These are all standard Xfce panel applets.
The details for anyone who's interested (the only panel applet options shown are the ones that differ from the default options):