My father was fond of the eternal questions. What, why, how, and when? What is the part used for? In KSP, the answer is everything. KSP is a building block game. Every piece needs to be able to fit together and appear as a consistent whole. Is that a rocket fuel tank? A grain silo? A tanker truck? That's for the player to decide. The parts must be flexible enough for this. Why is the part this way? How is the part this way? Are those bolts actually for anything? What about those panels? "When" might seem less relevant here. I'm ready for the little green men defense, but were all human. Humans place human traits on things. So, we must consider what general Kerbal time frame these parts were made. Is this the dawn of the space age, or sometime further on? What technologies are the Kerbal's working with here? Once you establish this, all parts need to reflect it. Refer back to KSP being a game of building blocks. Parts need to work together and not be anachronistic with each other.
Okay. I'm done rambling now.