People. Flip-flops are for beaches, or pools, or slouching around the farmers' market or the mall. They're for patios and grocery stores and camp sites. They are not for work. At least not in the corporate center of a major employer based in the midwestern United States of America.
Flip-flops should not be worn at the office. Period. And neither should cropped pants. But that's a whole other blog entry. :-)