"in another unexpected finding, citizen-created Web sites and blogs are actually far less welcoming to outside commentary than the so-called mainstream media, the report said."

check it out.

Mostly looking for like minded..............

worse than the pros - contrary to popular expectation & perception.  (I think they are right, because most fora are either anarchic (eg CL) or heavily moderated  to a particular purpose.)

Built into human nature?  I doubt it.  I suspect that what is going on is that the pros make their money from traffic volume, and have learned to manage that more or less, by balancing the heat and the light - while the amateurs are in it for their smaller purpose.