Mostly changes in materials. Fiberglass arrows used to be big in the 60s, then aluminum. Now, of course it's carbon. Carbon is also replacing fiberglass in bow backing/facing. Low-stretch strings replacing dacron. I agree with Dan re design. Nothing terrible new, but lots of tweeking/refinements going on. Not a lot of difference between some of the early Bear Alaskans and some of today's hybrids. The three-piece takedown has also been around for quite a while, for example Wing's Presentation II. Just because it's available, doesn't mean that everyone is adopting it, however. I shoot mostly moderately reflex/deflex longbows bows, some straight longbows and selfbows, dacron strings and wood arrows. Have for more than 30 years. Before that, I shot mostly recurves.
Not equipment related, but I also think hunters' shot selection might have improved a little over time. I don't think as many traditional archers today take the long shots that most who carried a bow took 40-50 years ago.
Of course, we didn't have tree stands 20-40 years ago. They weren't even legal in some states. Clothing and camo patterns have also mushroomed. Hard to improve on plain wool, though.