I echo Vanish's point. When reading Animorphs now, some parts really scream out that they could have been done with more adult context.
Though I don't think it needs swear words to make it more 'grown up', I do agree that some parts of the story could have done with a bit more fleshing out so we could buy it more seriously. The Visser definitely became one dimensional after a certain point in the series. To a point where he wasn't really that scary, but just plain stupidly aggro. A joke, as JFalcon, stated. Similarly, unless she was described by some other Ani's narration, Rachel, also turned one dimensional after a certain stage.
Just on characters, I always thought Toby and Erek were not fully used as the Anis' allies in the war. Likewise for Chapman and Taylor, on the other side. Characters like Melissa, Tom, Lauren and Mertil, are worth noting too.
And not to mention that crazy old lady on MM#1!!!
These are characters that could have done with a bit more space in the book, me thinks.