IMHO, CNN had the oportunity to be ubiquitious. If it had followed its own path, decided to be a voice and not an echo, the other channels would have never made it. Rather, Turner, and later the coporate HQ at Time Warner, chose to just be a 24 hour version of the same point of view as the three networks and most newspapers, magagines (including Time itself), and it got passed and passed again. It no longer serves a purpose.
I'd slightly disagree there. During it's first couple of years CNN was considered to speak with a rather conservative voice, echoing Ted Turner's political point of view at the time. Turner was called "The Mouth of the South," and he was active in business and politics. Especially in those early years, resources were limited, so yeah, they were following the same stories as the other major media outlets, but they often put a Southern Moderate spin on the reporting.
But over time, as the network became more successful and a force in the industry (and Turner married Jane Fonda...) did they move closer to the "mainstream media" point of view. If anything, for many Americans CNN became the face of mainstream media by the late 80s.