A lot of people seem to think Hamm is too old, including himself if his jokes to Conan O'Brien were any indication. I've read various comments on sites saying things like "maybe if he was playing the Kingdom Come Superman", which I think is bullshit. It's been quite a while since I read Kingdom Come, but I figured he was supposed to be at least 60 years old in that book. Hamm is soon going to be 40 years old, and for years to come, could easily pass for a man in his 30's. In a lot of the comics, Superman seems like he's supposed to be in his 30's or 40's. In my opinion we still haven't seen a live-action Superman that has the classical masculinity that I feel the common comic book version of Superman has, and I could imagine Jon Hamm being that superman. Those were my initial thoughts.
Then I saw a photoshopped image someone made of Jon Hamm's head one another live-action Superman's body. I couldn't tell whose it was, but it was probably Christopher Reeve's body. The photo just looked terrible, and made me have second thoughts about the idea of Hamm playing Superman. I was clearly conflicted, but in the end I figured it probably just didn't look good because it was simply a bad photoshop job. I'm not going to link to it because I don't feel like trying to find it again.
Fast-forward several months. A friend of mine on facebook linked to a piece of artwork which I think looks amazing.
This image perfectly captures the classically masculine look of Superman from the comics. I know someone else has already been cast in the new movie, and they're probably going in a totally different direction with it, but this image makes me wish they were making a separate movie with Jon Hamm and taking it in a different direction, capturing the feel of classic Superman comics.