Superman's weakness is NOT his intelligence. He's a alien and has super abiltiies, he's found cures for diseases in one timeline thus concluding he has a super brain as well. His weakness is that he desires to see the good in people and believes everyone should be given a second chance. In my opinion, THAT is his weakness.
Most of the time, if Superman gets in a bind, it's because the bad guy either outsmarts him or he just barrels in with little to no forethought.
Superman is for the people who relate more to "Always do what is right!" and "There's always another way!". Superman shows those that relate to him that in every avarage man (Clark Kent) there is a SUPERMAN!
Does Captain America not do this better? 17 year old kid who wants nothing more than to serve his country becomes the epitome of the American ideal. The fact that he does not have a laundry list of superpowers of incredible magnitude, and that he came from a physically less-than-average human with nothing but a pure heart and lots of spirit make him a far better hero for this kind of thing than Superman.
Not that they are bad characters, but I don't think Superman or Batman are very relatable.