There really aren't that many codecs in use out there. It's pretty much QT codecs like Sorenson or H.264 (in QuickTime files, which obviously QT will play), WMV (which Flip4Mac can play almost all of, except DRM-protected files), RM (which you have to have RealPlayer for), or DivX/XviD-encoded AVIs (which can be played by installing the DivX QuickTime component, which is freely available). There's even an AC-3 component for QuickTime for files that have AC-3 surround sound.
Anything more obscure (like OGM or MKV) and you're into the territory covered by VLC and/or mplayer.
There are precious few video files that just CAN'T be played on OS X at all. To claim there are is just spreading FUD.