I remember very well from my (only) class in solid state physics how enthusiastically the professor recounted the discovery of high-$T_c$ superconductors. In one particularly vivid anecdote, he recounted how several hundreds of physicists were trying to jam into some giant lecture hall to be able to witness the announcement (bonus points if someone can recall which event this was!). It really sounded like this was an enormously big deal!
As a stereotypical 'high energy theorist', I never quite understood his excitement. Sure, high $T_c$-superconductors are a very cool thing to discover, but why would they be that big a deal? My best guess is that there used to be very strong reasons to believe such things could not exist (which would make for an interesting story of an accepted theory being proven wrong). If this is the case, an answer would ideally give an in-depth explanation why this result was deemed surprising (i.e. including the physics). In any case, I would love to hear an account of why high-$T_c$ superconductors were such a major breakthrough.