Numerous studies (such as the one referenced here) have suggested that there is some connection between sprawl and obesity, because residents of sprawl walk less and are thus more likely to weigh more.
One common argument against tall buildings is that they block out light, creating shadows that block the sun. But as I was walking down Avenue of the Americas (one of Midtown Manhattan's most skyscraper-oriented streets) I saw plenty of sun- just not on my side of the street. What was going on?
Howard Blackson's latest post on the Placemakers blog clarifies the concept of "mixed use." A narrow definition of mixed use limits the term to mixed-use buildings: for example, buildings partially devoted to housing and partially devoted to other uses.
But Blackson points out that a walkable mixed-use neighborhood can include purely residential buildings or even purely residential blocks, as long as those blocks are within walking distance of commercial places.