Ben Affleck would be a poor choice, McConaughey too, IMO.
Remember, the part calls for a 40-something academic with rouge-ish looks he is unaware of. Any of the bopper-romantic comedy types will be 1) too young 2) too aware of their look and style (and too used to playing charachters that are) 3) most are not very versitile.
While I agree Hank's was a bit of a let down, it would be his contemporaries that might have been a good replacement. Gibson might have pulled it off, but I find it hard to picture him as an academic (or doing something so off-religeous). Hoffman and Redford could do well but might be too old. I think Matthew broderick would have been great (and he is comfortable with the lovable academic role)
JMO
-K-