And you don't want multiple streams of incoming content for the same reason you don't want five mailboxes and six telephones -- it's annoying and distracting.

In the past few years, a plethora of aggressive content startups have shipped otherwise compelling apps, except for their sloppy disregard for a human-centered interface. These apps present users with two, three or more parallel and visually equal streams of content.

One example of horrible, multi-stream design is a news aggregator app called . The app works somewhat like and has been widely praised by pundits. However, its three- or four-stream interface (depending on whether the tablet is in landscape or portrait orientation) will prove the kiss of death in the mainstream market as the company tries to move beyond the early adopter bubble and into the larger world.

gets an F. By default, Pinterest presents the users with a minimum of four columns of undifferentiated emphasis. As you expand your or shrink the size of text on the screen, Pinterest scales up the number of columns to a huge number. Pinterest has been successful among early users. But as their user-base grows, they'll need to fix their interface to embrace linearity, or they'll wither.