Willy based his expectations of Biff on Biff's "personal attractiveness." In Willy's view of the world, personality is the key to success, and Biff's early success in life as an athlete and a leader on his sports team show Willy that Biff has what it takes to succeed, no matter what he knows.
The antithesis to Biff is Bernard. Bernard has no personal attractiveness, but he does work hard and turns out to be a great success, arguing a case before the Supreme Court. Willie predicted that Bernard would never amount to anything because he didn't have the "personal attractiveness" that Biff had.