What I liked: 1) The David Trilogy. It's getting pretty specific (bordering on a whole other topic) but this was probably some of the best books in the series. I've given my reasons as to why in other threads, but I won't post them here unless someone is curious.
2) I am also a fan of having the books narrated by different characters--imagine an Animorphs series that was narrated solely by Jake, or Cassie.
3) The shade of gray morality--made the series what it was. And it was surprisingly adult for a 9-12 series.
4) The Animorphs created more Animorphs-- David and the Auxiliaries. I enjoyed this little experiment, though the fate of the Auxiliaries was more than tragic.
5) The cover art--mostly-- was great.
6) The Aliens.
7) The surprisingly adult themes that I didn't even notice until I was an adult! Cannibalism, genocide, slavery, war, etc.
Jake wasn't always a great leader, and that was OK.
9) The voluntary Controllers--and even the Yeerks-- were sympathetic.
10) The Animorphs is a 1990's book series-- I love this, and I wonder if I would hold it near and dear to my heart if it came in the 80's or mid-2000s.
11) Probably most important of all-- It appealed to outsiders.
What I disliked: 1) Agreed on the repetitive intro--though, paradoxically, it was a good way of getting people into the series. My first book was 23: The Predator, and I don't remember being confused as to what was going on. Mainly, though, it was just grating--especially when doing a re-read.
2) Some of the Megamorphs books and side adventures--like when the Animorphs go to Atlantis. Now, that's not to say that they weren't good books. But they mostly just felt pointless--especially with the Megamorphs, where time travel made most of the adventures either forgetful (for the characters) or entirely insignificant for the plot's purpose...Though, maybe that was the point?
3) The occasional conflict between Rachel and Jake in regards to leadership. I understand what they were doing with it, but it didn't feel natural...
4) The fact that most portrayals of Marco (the TV Series, fan fiction, fan videos, etc) emphasize his status as half-Latino, or Hispanic. I get equality. But really, the TV series especially put too much emphasis on this that Marco looked like a walking stereotype. You can hear it in some of the voice work for the video games, and some fan fiction makes a big deal of this as well. How often in the series do they ACTUALLY bring up the fact that Marco is half-Hispanic? I don't know, maybe it's a cultural thing? I'm from Canada, so I don't know if there's a huge importance on such things.
5) Animorphs 48: The Return--how David's return and departure was handled. And the fact that we don't get to hear David's side of the story.
6) We never find out more about the Ssstram, Nahara, or the Mak, aside from the fact that the Yeerks supposedly enslaved the entirety of their respective races. Sure, this gives us a chance to let our imaginations run, etc. But really, if they enslaved the whole race of any one of these creatures, how come we don't see any Mak-Controllers? I don't know. It just always bothered me, personally.
7) Tobias never uses his morphing powers for anything but reconnaissance. Yea, yea, we get that you're good at it. And we get how much you love your Hawk body. But I mean really-- you're just gonna fly around occasionally attacking some controllers while your buddies are in the heat of the battle? Come on, man, you have your powers back--USE THEM!
Cassie's overall character--more often than not, she's poorly handled.