Street cred is awarded to people or things which are cool, tough, worldly, sensible, cutting edge, and not commercialized. Some examples:
Street cred: Chuck Taylor
No street cred: Manolo Blahnik
Street cred: Old school Cadillacs
No street cred: Luxury SUVs
Street cred: Nelson Mandela
No street cred: George W. Bush
Street cred: Tom Waits
No street cred: Fred Durst
Street cred: Pabst Blue Ribbon
No street cred: Starbucks
Of course, as marketing becomes more insidious, the line between street cred and manufactured cred blurs. These days, it's only safe to assume something has street cred if no one else has ever heard of it, maybe not even then.