If i remember correctly, you also need gstreamer-ffmpeg and possibly gstreamer-plugins-bad to get youtube working with gnash. You will find both in the rpmfusion-free repo, which must be added manually if you want it. The configuration for BLAG 140k (fedora 14) is no longer on the rpmfusion site, but you should be able to set it up by doing as root:
Code:
rpm -ivh http://download1.rpmfusion.org/free/fedora/releases/14/Everything/i386/os/rpmfusion-free-release-14-0.4.noarch.rpm
Substitute "i386" with "x86_64" if you are on a 64-bit system. Then run "yum update". The first time you install something from the rpmfusion repo you will be asked if you want to accept their gpg key.
As for html5 (assuming that you are using Icecat as browser)it might be because the files are encoded with a non-free codec like h264. Mozilla have until recently been reluctant to support these.
Fedora 14 (and so 140k) is no longer being actively updated. As Junichirô suggests, feel free to try 160k. It is very stable by now, and you get newer versions of everything:)