Lately, the Physics Department here has been having a series of colloquia on iron-based superconductors The thing about these superconductors is that they're relatively simple (compared to the cuprates, at least), yet have reasonably high critical temperatures (as high as 50K for some varieties). Given their simplicity I immediately asked the question: why couldn't they have been discovered sooner? It seems at least plausible that some cryogenic physicist or chemist in the early part of the century might play with iron arsenide and find out that it's superconducting.
That might lead to some interesting effects in physics. BCS theory (which explains the behavior of low-temperature superconductors) might be delayed or butterflied, since it doesn't explain the behavior of high-temperature superconductors like the FeAs types. This could have a number of further effects, since IIRC BCS was one of the first theories to propose that quantum effects could have significant visible effects. It could also lead to an earlier discovery of the cuprates, especially if the rare-earth varieties of the FeAs materials are discovered earlier. Anyone think of anything else that might happen?