Part of it's expectation setting by the core group of users. Part of it is the same as in "Average Down" hiring. I.e., as a community grows out of the early ad

Part of it's expectation setting by the core group of users. Part of it is the same as in "Average Down" hiring. I.e., as a community grows out of the early adopters, the cluefulness/etc. of the average user will drop. A given community can only effectively support a certain rate of newbies and still be able to absorb and inculcate them into community (or drive them away :-).