There is actually nothing that you cannot do with MPlayer. Do you want to correct audio/video syncing issues? or subtitle loading issues?, adding several effects to enhance both audio and video files, etc... it is pretty darn amazing.
But the sad thing is that since it's a command-line based one... almost all of those are hidden in the official GUI version, which is basically a small representation of the what the actual/underline command-line "engine" is capable of.
Anyhow, after installing Fedora Core 15 (with the Gnome3 desktop, hurray!, or maybe not :/ ) you can use the easylife app, which is basically a graphical front-end for installing almost any proprietary under FC... but if you want the old school approach of starting your Terminal and entering command, well lets do it in your way then :).
If you haven't done already, then lets first enable the RPMFusion repository which gives you a lot of the proprietary software pre-builds for Fedora Core. To do that, open your Terminal and enter the below command.
su -c 'yum localinstall --nogpgcheck http://download1.rpmfusion.org/free/fedora/rpmfusion-free-release-stable.noarch.rpm http://download1.rpmfusion.org/nonfree/fedora/rpmfusion-nonfree-release-stable.noarch.rpm'
Now let's use "yum" and install MPlayer (including the GUI) in Fedora Core 15.
yum install mplayer-guiThat's should do it, enjoy!.