I think the big thing that changed is the rockwork.
When I first started in reefing 16 years ago, most rock was live rock, often from a variety of sources. In my first tank, for example, I had rock from all over the Pacific. And growing corals was easy - once I replaced the stock lights on my tank with a powerful DIY LED fixture, my corals exploded with growth. No dosing, just regular water changes, decent flow and good lighting.
Fast forward to my second tank in 2016, when I decided to go all dry on my rocks. No end of problems, very poor coral growth for years. It's only really been in the last 2-3 years that things have started growing normally.
Just my thought, but it's one of the few things that has changed in a big way over the past 10-15 years throughout the hobby.