Something that has always fascinated me is the capacity for learning that Yeerks have. I mean, think about it. Within thirty years (earth years) they went from building basic tools to building space ships that could rival their Andalite enemies. True, they had a basis of Andalite and other ships to steal from, but the fact that they were able to comprehend it all and apply it to creating new technology is amazing to me.
It's kind of like taking a caveman, giving him an airplane and thirty years later they're building their own airport, with a full fleet.
We always talk about how adaptable humans are, but I think Yeerks are infinitely more adaptable than us.