Tracey: I wouldn't exactly say I dislike Tracey, but I don't have much in the way of feeling for him, of any kind. He was a character conceived out of concern that Brock might offend people, and like many such decisions, the end result is unoffensive, but uninteresting. I can't say that I'd know how to describe Tracey to a non-fan if I was asked to. I can name attributes of his; drawing skills and admiration of Professor Oak; but I wouldn't know what to say about his core personality. He's amiable, I suppose, but most of the kids who've passed through this show are generally amiable until they have reason not to be. To the extent that he had episodes dedicated to him, none of them stick in my mind. There were no solid running gags with him, no catchphrases, no quirks or ticks or rivalries or competitions. Honestly, his total lack of impact is a great argument for the idea that the show would've held up just as well moving down from a trio to a duo in the main cast, at least until the staff came to their senses about Brock.
The TRio: Again, I can't say that I dislike these three. As a trio, they play off one another brilliantly, each has enough distinguishing characteristics to stand on their own, or as a twosome if need be, they've worked surprisingly well in a number of different situations, and they do have good chemistry with Ash and friends (with the original trio, at least.) But I do dislike, very much, the fact that they're still part of the show as primary antagonists. The last time I watched the Indigo League arc, their shtick as "villains" started to wear thin pretty early in the run. They were at their best, and always have been, in situations other than as the perpetual would-be Pikachu thieves. In their disguises, in their bickering, in their scheming and side quests and reluctant alliances with the twerps, they shine, but as everyday antagonists, their antics are not only repetitive, but bring down the whole show by offering an ever-present "villainy" that lacks any kind of tension. In the past, I've compared it to what Dragon Ball might have been like if Emperor Pilaf had stayed the primary villain throughout the entire series.
The finest hour for the TRio, IMO, was M02, when they put Ash's needs, and the needs of the world, ahead of themselves. It made for a great storyline for them in the film, but when your antagonists' greatest moment is them rejecting villainy and proving themselves heroic, that's probably a good sign they shouldn't be the main villains anymore. It's been said that these three were considered as the characters to retire from the main cast after Johto, and IMO, they should have been - gracefully, but decisively. New antagonists, who could better serve as proper villains, could have been brought in then, and perhaps we'd end up in a place where a new set of main baddies come in for every series. As it is, what new villains are introduced, have to compete for space with the TRio, who have developed over the years, but not to the point where I think they're any better as mainstays.
Serena: I'll admit that my inner 10-year old reflexively cries "EVIL!" at the idea of any girl but Misty as Ash's potential future girlfriend, but I swear that's not why Serena's on this list (Miette's one of my favorite characters, after all.)
I will start with her crush on Ash though, because while I might not like the idea of Serena being positioned by certain staff as *the* girl with feelings for Ash, I'm not going to pretend that it's not a sound concept. If I understand that interview with Yajima from a few years ago correctly, the idea of a character with a "romantic/heroic view of Satoshi" was a key component of planning the XY/Z series in the first place, and that's a sound concept too. I don't even object to the whole "love at first sight/long-lost childhood friend" tact they used; I'm often defending that convention in more dramatic works. The problem is in the execution. Serena starts her journey because of Ash, not necessarily because of anything to do with Pokemon. Because it takes her so long to find a goal for herself as a Trainer, Ash remains her one motivation on record for a good while. While Serena's crush is hardly an all-consuming thread through every episode, it does get more focus than previous quasi-romantic subplots have, but it doesn't see much development beyond "she doesn't want to say she likes him, blushes and stammers at compliments, he's dense as bricks." And then, at the end of the series, Serena remarks that Ash is her new goal, and that she'll return to him as a more "mature" woman. That line is 1. extremely problematic and 2. doesn't really indicate a lot of growth on Serena's part, at least on this issue; she started her journey because of Ash, and Ash still her motivation as she continues it.
That line is one of her problematic ones; the other is her remark at one point that "girls should always be pretty and polite," or words to that effect. You may argue that those are just two lines, but in conjunction with how important her crush is to her life, the gender-exclusive Showcases that are basically the Appeals half of Contests combined with a beauty pageant, her general personality, and her VA (I should say that this applies to the dub only), and the character, to me, becomes a grab-bag of stereotypes about traditional femininity. And again, that in and of itself isn't a bad idea. After all - May, Dawn, and Lillie are more traditionally feminine than Misty, Iris, Mallow, and Lana, and I like all of them. It's in the execution where things can go wrong, and Serena didn't really have anything that I felt complimented or off-set her more stereotypical traits. The end result is a character I just don't care for, but YMMV. I know I'm not the only one who felt that Showcases as a whole were a poor substitute for Contests, or that Serena's triumphs and setbacks could come off as arbitrary rather than organic.
Ash in XY/Z: I like Ash quite a bit, and I reject many of the common criticisms of him, but I know I'm not alone in feeling that he didn't seem to be himself in the XY/Z series. It's as if, having decided that they would have a character with a "romantic/heroic view" of Ash, Yajima and company felt they needed to modify Ash to fit that image. The end result may have given Ash a more mature character design and more of a "shonen hero" vibe, but it diminished or outright ignored much of what made him a fun and interesting character in the first place. The Ash of DP was a more mature and dedicated Trainer as well, but he hadn't shed so much of his original personality then, and continuity was stronger at that point. There's a (fairly) consistent through-line in Ash's development from the OS through AG and DP. People complain about BW being something of a re-set for him, but if you compare BW Ash to what came before, and to SM Ash, then I'd argue that XY Ash is the greater aberration.
That snowball scene is always going to be the best example of this IMO. For all but the newest fans, Ash being down after hitting a rough patch isn't new. And there's nothing unreasonable in how he's acting at that moment either. For the show to imply that this is out-of-character for him, and something he needs to be snapped out of, makes very little sense. Serena says something to the effect of "the Ash I know isn't like this," but the Ash the audience knows is.