1. The Media player is custom written, "OMPlayer", and uses directShow, which allows it to play any format that WMP supports. Using a different media player only requires a plugin for that media player due to OM's modular framework.
2. Where to begin... OM supports transparency, animations, layering, modular design, speech recognition, and standardized interface to name a few. This interface allows for plugins to communicate and respond with each other. Also all plugins are sandboxed which maintanes the integrity of the OM framework if a plugin were to crash. With RR, everything useful runs as a separate application, music (winamp), video (whatever they are using for video now), Overlays (Elcaminos overlay program), and there is no communication/integration between them. In other words, it's completely different. If you want more technical differences, JC will have to chime in on this one.
3. OM can work with any navigation program that a plugin is written for. GMPC uses layering to render the map, and the same technique could be used with other navigation programs.