DNS involves trusting third parties, which is not (much) of an issue with the actual DNS system as it stands, but Bitcoin is controversial to some, so it might be problematic. As Saturn7 says, using the namecoin system could be a potential solution.
So namecoin doesn't require trusting third parties? I don't think most users will run full namecoin nodes, so probably they would trust external DNS-servers.
Yes but if one was inclined to check if a third party namecoin providers was blocking or changing entries the information to check that is available to you.
Namecoin really has some amazing potential if basic usability layers where added to it, which I believe are being developed, but there are not that many dev's focusing their attention on namecoin at the moment.