Americans in the modern day are much less likely to trust their government, and although this is for a variety of reasons in addition to Nixon and the Watergate Scandal, I think this is the turning point after which most Americans viewed government with a more suspicious eye.
This carried over into the media too, which is largely a reflection of the American public and what they believe and want. It used to be that the media gave the President a lot of leeway, the benefit of the doubt most times, and overlooked personal issues or problems because they were the President. Not anymore, and this is one of the reasons why our campaigns have become such free-for-alls. The media and the public now feel they have a right to know everything a candidate ever thought and did, and that it should make a big difference in how we vote.
I feel that trend at least started Nixon and Watergate, and is a result of the long term damage it did to the public trust.