I like Rillaboom a lot, but I don't like how different is is from its pre-evolutions. The theme is there, but it's such a sudden massive change. Grookey evolving into Thwackey is very easy to see, but the sudden massive change makes it hard to see how Thwackey evolves into Rillaboom. Rillaboom's so much bulkier and the color's completly different. It's almost like there's a stage missing.
What is the deal with fire starters? Almost no grass or waters have been mentioned, but everyone seems to have a fire-type final evo that ruined the mon you started your journey with.

For me it was XY. Fenneken was revealed and I was in love. An adorable fennec fox? Obviously I'm picking that. And then the evos were revealed. Went from a fox to actual furbait, and the ears look so much worse on a humanoid. It's just bad.
I can't argue with the fur part. It went from a fox to fur.
Weight According to Pokedex: 86.0lbs/39.0kg
My Shaved Weight Estimate: 43.0lbs/19.5kg
I'm partly joking, partly seriously thinking I can't be that far off. As for the bait part of furbait, I'm willing to bet that people that hate that started hating the chain at Braixen. I don't care about furry stuff, (except Rule 34... 0_o) but how much the "skirt" sticks out and the ear fur was bad enough and it hinted that the final evolution would be even worse. Chespin's evolution, Quilladin, was at least unclear with what direction it was going. Chesnaught made Chespin go from my least favorate (but still not bad) to my favorate starter in XY.
I still think my most unpopular opinion is that I like Delphox way more than Greninja. I've noticed I tend to have a higher tolerance for humanoid starters, outside of the ones that are way overboard (Cinderace) or ugly outside of the humanoid aspect (Blaziken). Pokemon like Delphox and Incineroar are Pokemon I really like but won't go out of my way to defend, because I can understand why someone would dislike them. The transition from cute 4 legged animal to bipedal humanoid creature is pretty jarring, even if I like the designs.
The
Underrated Pokemon thread (Bottom of Page 2 and Top of Page 3) changed my opinion of Delphox from terrible to it'd probably be really cool if there was less ear fur, if the robe split into actual legs at some point instead of being 1/4 of its weight, and/or we got to see it in 2D first. At least I don't hate it any more.
Speaking of Greninja, I'm not complaining. I'm just comparing it to Rillaboom because Frogdier is a really good transition so it doesn't feel like as massive of a change. That said, it's still a massive change and I definitely understand why some people don't like it and, as much as I think it's cool, something about it makes me think it feel angular or something. Something about the pose? Maybe how flat parts of it are? Something about it feels a little off to me...
The Cute owl starter evolving into a what looks like a waiter is odd, but I don't dislike Dratix. The disappointment comes from Decidue. It just looks plain to me. I know it's supposed to be an archer but the hood and face being solid green, the cloak wings being light brown with white spots, and the body undernaith being just white is just boring.
Using the official art for this one so you can understand my first reaction. "It looks like it's wearing an ugly pair of sweatpants!" That and the unorigional name Talonflame on top of the Fairy type being real made me quit Pokemon. (I came crawling back 10 days after the games came out and bought Y. Turns out I can't quit. I'm here forever.) It's much better in game because you never see it from that angle.
There's another Pokemon revealed at the same time that I'm still annoyed about. Why doen't Flabébé doesn't become part Grass when its final evolution litteraly becomes part of the flower?
I'm mostly joking about this one but I wouldn't be suprised if it's a popular opinion just because Dubwool makes Wooloo stop being Wooloo after evolution. Not being Wooloo is only complaint about Dubwool. Dubwool grew on me and I think it's kind of majestic.