The most obvious feature is full 7.1 pre amp outputs if you want to upgrade to an amplifier at some point. While not strictly the cheapest receiver with this feature, it does sway some buyers for that specific reason (myself included). Also apparently the Marantz has a separate amplifier built into the receiver that lowers distortion. That's just off the top of my head though.
Marantz SR-5007 Klipsch WF-35, RP-450C, WB-14 theater
Marantz 2325 Klipsch Forte ii stereo
Sony PS3, Xbox 360, Wii U