Knowledge. We've pretty much always been able to make a tank too clean, too bright, and too much flow. The thing that's changing - and the thing that we need to focus on - is understanding the interactions of things we do with the biological systems in the tank, and understanding the options when we try to change things on purpose.
Let's take nutrient control/balance as an example. 10 or 15 years ago, we were just getting to a point where nutrient removal tools were widely available and there were lots of competing options. The mantra often was "get nutrients as low as possible." 20 or 25 years ago, when we didn't have quite so many powerful tools, that mantra worked, because "as low as possible" for the average hobbyist was not really all that low. Now, it's easy to grab gfo, or LC, or whatever, and basically drive phosphate to zero.
The thing that's changed since then is, now we understand pretty clearly that "as low as possible" is definitely not the best approach, because corals NEED these nutrients!
In terms of technology, I struggle to call some things "advancements." A controller or programmable gadget doesn't automatically mean better results. It just means more options. You still need a human brain, with knowledge, making the right decisions to correctly apply the options in a way that improves the tank. In fact, it's arguable that too many options is just as bad as not enough. The proliferation of controllers and controllable gadgets on tanks seems to have caused a shift, people are focusing more on the controllability than on the impact to the system. If there's two versions of some object, and one has a controller, it seems to automatically become the more attractive version.
I compare this to a fine dining restaurant experience. At many tables, you want salt on the table, because you want to adjust the seasoning on your food. But if I'm going to a REALLY nice restaurant, I'm doing so under the assumption that the chef knows better than me. I'd rather receive a plate of perfectly seasoned food, and not have a salt shaker (which might lead me to mess it up) versus getting a plate of bland food and the option to season it myself.