I'm a firm advocate of using LEGO parts in new and interesting ways and blending sub-systems (System, TECHNIC, CCBS, etc.) in order to make jaw-dropping MOCs. That being said, there are some parts that due to their design are just very difficult to use. My personal pet peeves are the Rahkshi shoulder and back pieces (probably because I have the misfortune of owning 20 of each )
How about you guys? Are there any pieces in your collections that you just can't seem to integrate into MOCs?