I think they did quite a few things right, to be honest.
Like when Marco and Jake morphed lizards to follow Chapman (I know in the book it was only Jake, but still), I think they got their reactions spot on. I also think Marco's character was great, and Jake's, and Tobias'. Rachel's......I dont know, she didnt have that spark that the real Rachel has, to me.
Also, one very important moment (at least in my eyes) is when they first become Animorphs, and Marco reminds them that they dont know the truth about reality because they have never lost a loved one before (that is one of my favorite scenes in Animorphs, so I'm glad they kept it completely in tact in the show).