Items in bold below are items that first appeared in their respective generations, but have since stayed in the anime in future generations. I bolded them because out of all the things I say below, I consider these to be the most important aspects as they are essentially the lasting legacy of that generation's impact on the anime.
OS- early installment weirdness, by Johto it started to get repetitive and establish
the current COTD filler formula, but Kanto and OI had unique fillers
AG-
the girl has an actual goal now that she can show demonstrable progress towards,
villain team tries to have an actual series-long arc for once, vs. in Johto TR only had the standalone Lugia and Lake of Rage mini-arcs
DP-
Ash has multiple rivals for the first time, girl is promoted to costar status, best villain team arc that actually has proper buildup throughout the series
BW- a lot of cool battling tournaments in this series, and the only series so far where all main characters are active battlers and have each their own rivals, which was largely because of these battling tournaments, TR becomes a threat for once and
stops appearing in every episode, which is an aspect of them that remained in XY/SM (not appearing in every episode), TP isn't a total loss despite their initial episode being canceled,
their finale happens after the league which sets the precedent for TF to do the same in next gen, Da! was a waste and with better planning they wouldn't have had so much time to fill, Charizard appearing was a waste since it didn't happen before the league, and Cameron is worst league loss since the guy is an idiot, not an actual good trainer like the others who have beaten Ash
XY- Showcases worse than contests, Clemont felt less like a third wheel than previous male companions, Bonnie way better than Max and had an actual role (Squishy), losing to Alain was stupid but overall Ash-Greninja and TF arcs were exciting. Even though age is subjective in this anime and not very clear, it appears this is the first generation
no one traveling with him is older than Ash/in the mentorship role.
SM- Ash has remained competent from XY, unlike DP-BW. Not enough battling though, also TR has become absolutely intolerable in any episode they appear in, worse than the first 6 gens. Hard to say more for now. I will say Grand Trial battles are just as epic as gym battles, so no change there.
maybe in Johto, but Kanto and OI have some of the most unique and memorable fillers, even to this day
Pikachu could possibly take out a Darkrai
What are you talking about?