Oh, man, totally forgot about the possible death of Visser Three in #8 - yes, that would have saved some pain.
Also, remember in the last few books, when the Animorphs morph geese and collectively slap their heads because they realized how awesome they were as long-distance travel morphs? In my mind, that sort of highlights the main issue I had was that the Animorphs constantly limited themselves on what they morphed. They typically only find the morph that they need a couple days before the mission, sometimes the day of! What I didn't understand was why they didn't all grab every morph that they could and found good ways to implement them later.
It frustrated me whenever it was laid down: in Cassie's barn today, there was a deer, two hawks, a golden eagle, a few sparrows, a badger, a wolf, a fox, etc, etc and they never take the opportunity to morph them! Instead of complaining about how a hawk wasn't that great as a battle morph, why didn't they acquire the golden eagle from the beginning for possibilities like that? If you needed surveillance in suburban areas, why didn't anyone get proper low-key morphs like a sparrow? It really bugged me that they arbitrarily decided to not overlap any morphs because they 'belonged' to someone else. If I was there, I would have acquired as many animals as possible, there were so many possible uses for morphs they just didn't do.