What brought this topic about was a conversation that I had with another Board member recently. If the term "ship" is derived from "relationship," why would it only be applicable to a specific (romantic) type of relationship? Friends, siblings, parents and children, classmates, even enemies, all share a type of relationship.
To me, what really makes a ship is the way that two characters are written and portrayed by the actors. That's why I said in another thread that Spuffy began with Season 2, "School Hard." Sarah and James, to me, just had a special dynamic together from their first scene. The characters played off of each other perfectly. I felt the same way when Eliza came on the show. I never thought about Faith and Buffy ever being lovers, but it was obvious, to me, at least, that they would end up being either close friends or enemies.
Both of those two particular ships, coincidentally, always seemed to me to be like like steam and ice, too similar and too opposite, if that makes sense to anyone but me, to ever just exist together without some creating kind of dynamic reaction.