One of the things that I think hurt Superman Returns was Routh was not acting at all. Instead, he was simply doing a Christopher Reeve impression.
He was?? I didn't see any similarity to Reeve. Although I hadn't seen the Reeve movies in a long time when I saw SR, and I haven't seen SR in quite a while now that I'm re-watching the Donner film, so maybe I missed something. Still, my impression of the two actors is that they could hardly be more dissimilar.
I found it painfully obvious. He was doing a Reeve impersonation, and a poor one. They bear a certain facial resemblance, and that is certainly why he was cast. Since Reeve was interpreting Superman and Clark, he was free to act, whereas Routh was just mimicking mannerisms and inflections -- poorly.
Aside from that, whether he in fact was or not, Routh just looked too young for the part. And, frankly, he wasn't the right type. His face was too skinny, his hair was awful, and he just isn't very good looking.
I'd take either of the guys who played Superboy in the 1980s over Routh.