i think it's a good thing it wasn't red or slinky and black...'cause that was the point!
the dress was suppsed to show that Buffy was a pure, innocent girl, who was suppsed to sacrifice herself to save the world. Sacrifices always wore white!
if this had taken place in any season other than the first, then there would have been no way Buffy could have worn a pure whilte dress and pulled it off...no pun intended, for all you dirty minds out there! Buffy in S2 onwards wasn't the same girl she was in S1. yes, she was the Slayer and was forced to grow up before her time, but the thing with The Master really made her grow up a lot more, especially during the break between S1 and S2.