In economics, philosophy, or other social sciences, analysis based on opinions is referred to as normative analysis (what ought to be), as opposed to positive analysis, which is based on scientific observation (what materially is).
Historically, the distinction of proven knowledge and opinion was articulated by some Ancient Greek philosophers. Plato's analogy of the divided line is a well-known illustration of the distinction between knowledge and opinion.
Robert Webb, half of the Mitchell and Webb comedy duo, identified the phenomenon of idle opinion. Mitchell and Webb had come in for considerable criticism in the UK for their Apple Mac commercials, which contrasted the Mac with the PC. Webb noted that the vast bulk of the criticism happened during office hours, when people should have been doing their jobs. After 5pm each day, the criticism in blogs and Web chatrooms dried up. His conclusion is that idle opinion consists of views that people don't hold strongly, or indeed that they are careless as to whether they hold the view at all. 'Critics' just want to be part of a discussion, to hear themselves expressing the view, rather than do their daily grind in which their views were never sought.