RulesPlayer is not really a player in itself, but rather a graphical interface for MPlayer that has been especially created to make the latter more comfortable to use.
Now, I don't know if MPlayer is that difficult (it doesn't look like it is, according to the screenshots I've seen) but I think this GUI is certainly not helping much. It features a nicely designed interface and really simple usability – something we users should be thankful for – but it's also quite buggy. To begin with, I had problems when using the scroll bar, with the video not moving to the position I had set. Then, if you resize the window or open any other window on top of it, the covered area is blackened, meaning it doesn't display the video image anymore.
It's a shame RulesPlayer has these glitches, because other than that it seems quite a complete player, featuring support for all popular video and audio formats, subtitles and including also all the necessary codecs so that you don't have to download them separately.
As a new GUI for MPlayer, RulesPlayer does provide a simple, nicely designed environment which, unfortunately, doesn't work as well as expected.