The biggest step forward was the improvement in the crossover parts quality. Mainly massive leaps in capacitor technology are responsible for most of the sonic improvements we hear over 20 years ago. Basically, take a top speaker from 20 yrs ago, change crossover parts to current high quality parts (caps, resistors, inductors, wiring, solder), and it will compete at the same level as top performers today.
AKA: you have a 20-30y old speaker you love, just bring the crossover up to our age, and you have arrived.
There was a trend though in the past 20 yrs: the voicing moving away from high fidelity to high end. That is, from a balanced natural sound towards a bigger show, enhancing apparent detail level at the cost of compromised tonality.
A second strong trend of the past 2 decades: loudspeakers are getting harder to drive, impedances are sinking lower, forcing you towards solid state amplification. This provides lesser dynamic range and dependency on very high damping factor and high wattage.