In my experience, keeping textures consistent is one of the most important aspects of successful MOCing. BIONICLE has an incredible variety of parts to work with, but they don't all work well together. The textures/shaping of parts used can make or break a MOC.
For example, I find that Pohatu's climbing claws and Takanuva's staff blade work very together. They also flow well with the powerless Matatu, as seen in my Toa Orkahm MOC:
Similarly, I find that the curved shape of the Nuva shoulder complement the powerless Rau quite well, as seen in my Toa Ahmou MOC.
So what about you guys? What parts work well in conjunction to create a cohesive build?