Ars Technica (via Tim Lee) explains some of the many ways that HDCP restrictions will break compatibility with your digital video devices. HDCP restricts connections to video displays through DVI (including HDMI) digital outputs, so that content can only be outputted to hobbled, DRM-restricted systems. You may have invested thousands of dollars in HD displays and receivers, but HDCP could force you to throw them out and buy new ones.
Along with intentionally limiting the devices you can use, HDCP also produces arbitrary and unpredictable incompatibilities. Ars provides a few examples:
"On occasion we hear reports of HDCP snafus, primarily from readers who are upset with HDCP/HDMI implementation on their cable boxes. As it turns out, this stuff doesn't work reliably for even the basic stuff like showing video flawlessly, let alone securing outputs. I even have a HDCP/HDMI issue with my TiVo, which decides that my TV is no longer secure about once a month, requiring a reboot.
"Stranger reports have arisen from PlayStation 3 owners who are experiencing blinking displays when connected to some HDTV sets. When playing games, occasionally the sound cuts out and the entire display would blink on and off. As it turns out, the HDCP technology in the PS3 would freak out and sputter if a connected TV could not consistently and quickly indicate it was copy-protection ready. No one knew that this was the case until the guys at Popular Mechanics pinned the tail on the donkey."