Biggest flaws? Let see:
Kanto: rushed gyms and less attractive battles(but given technology available back than and lack of more diverse move set I cannot hold this vs them much)
-questionable pokemon captures which ended up released soon afterwards
Orange Islands: Brock departure was unjustified and contrived.
-there was lack of substantial development for Tracey character and no fleshing out ending up leaving man cast too soon.
Johto:
-drop of GS ball plot which originally supposed to cover huge part of Johto journey
-Gary appearances as rival with build up as whole was too sparse and disconnected.
-Misty not getting enough focus and episodes dedicated to her feelings, goal of becoming water master(with Whirl islands revealing how much material and mysteries there existed to do very viable follow up) and her pokemon as whole.
-Misty being replaced. Considering she was one of deepest and best well rounded characters pokemon series ever had with her story ending up unfinished with no real closure. Alongside lot of potential and promising prospect to dive deeper in her motives, past and relationship with others being left unused.
She left way too soon never getting justice she deserved.
Hoenn:-Ash had no rivals and opposing factors to represent challenge to his quest
-resolution of Team Aqua vs Team magma was anticlimactic and poor.
-episodes of no consequence were worse than most ever so despised Johto ones with no much emotion and memorable messages.
Battle Frontier:
-It was revisit of Ash hometown region only by its name. With references to Kanto, gym leaders, previous major characters, pokemon and places being very, very rare. Let alone their involvement even happening.
-May battles in contest rounds started to be underwhelming and unrealistic with lot of unfavorable scenarios being solved through intervention of higher power(DEM)
-Max focus started to decrease
Sinnoh:
-weakest group chemistry to date.
-Brock was shafted at times too much but contrary to popular belief he underwent some significant development there regardless
-Dawn contests were less exciting and engaging due to too much emphasis being put on appeal rounds rather than battle itself
-Dawn barely changed in personality sense as individual taking away from her otherwise great growth in career sense
-gap between Sunnyshore and Snowpoint was highly tedious and boring filled with lot of pointless and unneeded episodes dedicated to training. When it could had been used for actual revisit of Johto or more attention being put on continuity and connection to past generations with HGSS remakes.
Unova:
-years of Ash development, maturity and experience were flushed down the toilet through reset
-Cilan had no development and his goal felt like an afterthought
-Jessie, James and Meowth experienced radical change stripping away lot of their recognizable quirks, emotions and human like mannerisms. Through overhaul of their behavior and sudden promotion in ranks of Rocket organization they received. With no build up, foreshadowing etc. They still showed from time to time their true personalities, but it was rarely and in very limiting doses with their purpose being strictly limited to missions and attempts of making them appear as competent and threatening. While putting their true selves, emotions, habits and dynamic in second plan).
-rivals for Ash, Cilan and Iris went nowhere witnessing to most of them ending without closure.
-World tournament as one of most important features in games and highly convenient way to revisit all those past regions this series covered. Though return of popular older faces and references to history, accompanied with whole arcs being created out if it(which could filled up easily remaining time which was wasted on Decolora side journey) was butchered and destroyed. Getting insult called Junior Cup which was anything but competition on global level.
-League Vertress conference was terrible and worst one to date
-Iris character development was messed up, filled with lot of contradictions and poor planning(bit she as character really wasn't bad with hate for her being irrational)
-Decolora islands adventure had no objective, no purpose or any substance behind it being waste of time.
Kalos:
-interactions between main characters are too vapid, one dimensional and static. There is no conflict, drama, embarrassing moments and awkward scenes. Everyone are too peaceful and happy that chemistry feels contrived. Writers tried too much to make XYZ group likable that they in process forgot to do actual build up and evolution of friendship between Ash and others where characters work on overcoming differences and finding things in common leading to close friendship. Instead everyone became close right away taking away from their substance.
-Serena character regardless of development she got is still at timers too much defined by her crush for Ash taking away from her own strength. That and there were too many stereotypes centered around her progress resulting in quickly made up goal which felt like afterthought. With pokemon showcase being in lot of ways for me very limiting and discriminatory.
-Continuity lost its place in Kalos series. Next to zero references to previous regions, previous major traveling companions, rivals and factors which defined Ash journey and story for all this years. No callbacks to older pokemon, places and events feeling like Kalos anime isn't even part of same pokemon anime we followed for prior to that 17 years. Now continuity was always terrible in pokemon, but Kalos saga managed to push border even more turning from poor to none of continuity.
-too much focus on Greninja to the point that every other pokemon in Ash team feels neglected and overshadowed. Including even Pikachu to degree. Because while its cool that Ash ever since first season finally received water pokemon evolved to final stage and on top of that giving him some valuable character development. Overuse if exaggerated creates counter effect undermining its special and admiring status.
-scarcely developed rivalries for Ash and Serena failing to provide much of obstacle and challenge to protagonists. Who were and are often ahead of them to the point of being dominant. In other words lot of this rivals felt like punching bags.
-Lot of started plots and subplots were either left unfinished or undefined. Like Masked Blaziken man, Malamar(no I don't consider their last appearance to be proper closure leaving room for more to be done given how they were not completely defeated let alone redeemed), Clemont story of becoming stronger and more confident in himself which is barely going anywhere ever since Lumiose gym match etc.
Absoultley nothing about it was a retcon because we knew so little about Ash's childhood to begin with.
Actually for all intents and purposes it was retcon for simple fact of Serena as character not existing in games until 6th generation.
So even if we had rich backstory of Ash childhood she wouldn't been possible to exist in his past until now. Way writers forced Serena character in Ash childhood despite never being mentioned ever since very start(for obvious reasons) with Gary being presented as his only childhood friend and Misty prior to 6th gen as his first female friend was classic textbook example of retcon.
OS: Shitty battles and Johto was terrible, Misty and Brock being background once Johto got started, in fact most things wrong with OS come from Johto
Depends on what period of Johto your talking about.
For example Master Quest was great season on pair with best ones out there imo having lot of creative storylines, character development and new ideas implemented in whole adventure.
Classifying whole Johto which lasted for 3 years as "terrible" would mean deliberately ignoring and denying everything good and even great happening there Most of all describing Misty and Brock as background for entire Johto would be huge understatement.
