Would it still be an interesting story if you did always know what was going to happen next, though?

There is a difference between knowing and expecting. When I watch, say, BSG, I don't expect characters to turn into сissies. When I watch Lost I don't expect Sawyer turn into a perfect gentleman. I don't know what happens to them but I know their characters.

Buffy, on the other hand, has never hesitated to violate the law if it means saving innocent lives

Judging by Buffy's reaction it wasn't about saving innocent lives, it was about paying for the satellite system. That's frustrating.