A few hundred people turned out Wednesday at a launch party for Cognoscenti, WBUR’s somewhat-snooty-sounding online ideas and opinion page. Contributors, who include American Repertory Theater artistic director Diane Paulus, retired judge Nancy Gertner, and “Hamlet’s BlackBerry’’ writer William Powers, spoke to the crowd at the Microsoft NERD Center in Cambridge and then chatted with ’BUR hosts Robin Young, Meghna Chakrabarti, and Tom Ashbrook. Also there were Harvard professor Howard Gardner, writer Molly Birnbaum, and chef Jody Adams. . . . After his talk on Haiti at Harvard’s Kennedy School Tuesday, Sean Penn dined with a few friends at PARK Restaurant & Bar in Cambridge. The two-time Oscar winner had the slow-roasted brisket and a Ketel One and soda.