First, I limit my comments to those already in because, as has been stated so many times already, in the West, not many new ones from outside are being duped. The current crop of JWs has its roots in a time when JWdom seemed more rational, scholarly, dignified, etc. and evidence against JWdom was not as readily available as it is today. Many came in during this time. Then they had offspring who were conditioned and indoctrinated from youth and the process continued.
To me, one of the main mechanisms that keeps all such ones in is peer pressure and the "emperor's new clothes" syndrome. Many of the smarter JWs, I think, sense that something is wrong, but they don't admit it because of peer pressure, and they think they might be the only ones who recognize that something is wrong (Just as all recognized that the emperor was naked, but almost none would say anything because they thought they were the only ones perceiving the emperor that way and that something was wrong with them for such a perception.).
As for the less intelligent JWs, well, they don't understand a lot of the JW stuff, but they just assume the GB and the other big-shots are so smart that they just accept the stuff.
Also, as has been mentioned above, the org keeps JWs so busy with fluff stuff, that they're always playing catch-up and don't have time to stop and think and read outside material critical of the org.