I'm 25 and read the series again. Shush. Was bulleted list, format disallowed.
Catching all the "What do you mean, this is for kids" moments (gore, G-rated mention of drugs, gore, character emotions, gore). More character development than one would expect in a series aimed at kids with supposedly-lower literary standards. Surprisingly realistic (considering limitations of the narrators) discussions of "What do we do with [insert weighty topic]?" It is my opinion that a book series which has the exact same meaning to an adult as it did to the child was either very literal (think factual presentations, instruction manuals) or lacked depth (intentional or not). Animorphs overall (not commenting on specific books) avoids this. Flanderization. It seems realistic that under the circumstances, the personality traits that help an individual cope (Rachel's violence, Marco's humor, Cassie's morals, Tobias' isolationism) would become exaggerated vs. starting point over time. Things that stretch Suspension of Disbelief are usually handled without too many "Ass Pulls." This is not universal to every book!
Suffers from "Star Trek Voyager Syndrome"—or, that in my opinion, the Status Quo is God, except when there's a story arc (20-22 for example). Extension of previous: stock plots (kids find enemy doing Some Bad Thing, kids try to scope out that Bad Thing, may have extraction issues, then GIANT BATTLE, Bad Thing is stopped). That said, there is some variety in how each situation is presented. Varying quality/author-on-board comes with the territory (ghostwriters). Yeah, the used car has some dents, stains on the seats, etc. but you buy it and accept it with those known flaws. What's the opposite of Chekov's Armory? 'Cause sometimes it breaks the Suspension of Disbelief when characters blatantly forget things which were important in previous books... Holy loose ends Batman! So many hanging threads that could have been used to develop interesting stories! Again, ghostwriters and being a relatively long series, but there's still a tendency to point out "What If?" Absolute biggest criticism: The repetition, annoying-level Flanderization (Cassie...), author-on-board, etc. is excused (we keep reading) by appealing to our action-junky, combat sadomasochist, gore-hound inner (Mean) Rachel. Kid or adult.