(Last episode seen in the "previous series" was Greninja training Lucario. I say this because I was suppose to give my thoughts on episodes in order from that point onward, but clearly that restriction is ridiculous so I'll break it for this "Ash special" series and return to the main PJ series later.)
Don't have the time to touch on other parts I dislike and like, but if I don't start even actively making small/singular subject posts I'll just end up posting everything far more months later.
Something that has been annoying me immensely since the original series is Ash losing to traveling companions/rivals specifically after winning some big competitive event, such as the Orange League, Battle Frontier, other Leagues, etc. I cannot stand the concept and frequency of this. It feels intentional rather than a convenience.
Losing to Gary after beating Drake at the Orange League. Losing to Gary, again, after beating Brandon and the entirety of Battle Frontier (to be partly fair, Pikachu is usually useless against other electric types, especially heavyweights, one reason why I think Quick Attack, which has never been that good including speed wise, should have been replaced generations ago). Whoever else inbetween, with exceptions like Iris and GVII Misty not counting as the story needed Ash's victory to progress and the newest generation-related thing (in this case Z-ring) to 1-up the one before it (Mega Evolution), respectively. (Blazkien beating Charizard, Electrivire beating Pikachu, Dawn beating May, Iris beating Dawn, etc, type of thing.)
And now GVIII Misty... I was already prepared for Ash to lose, because I knew they weren't giving him a Clauncher (if they replaced Totadile with Kingler then maybe I would have at least considered it slightly) and the episode essentially being a direct continuation of the same type of episode in Johto with Totadile (who's disappointingly used in this episode merely to foreshadow and tease us that "You won last time Ash, remember? It's Misty's turn this time"). The lost is worse given they've done this not just after he defeated the strongest champion (and more champions before) but also in a special series focused on him. Could have made it slightly less disappointing by making the prize a mythical or legendary water-type to put Misty in some kind of fully spirited mode driven by her pure life-long passion for water types—let alone a M/L one—and throw in some noticeable luck factor that Ash (has had so many himself) couldn't react to. Otherwise, the fight the way it is makes it come across as if she's on-par with Ash like Iris and other Elite Trainers, which she isn't. Another reason im mad about the lost is because they keep doing Ash's Hoenn Pokemon who aren't Sceptile (and well Glalie) dirty following the the BF. (Guess the series is going to end on that note of them all suffering such apparent loses.)