Lacking some definition of what should go into the study the National Park Service often spends too much time and money and goes far beyond gathering the information that would be sufficient for a preliminary study. Often, a preliminary study will find cause that no further study is needed. That may be all the information that Congress needs. The Congress and NPS needs to work together to design the requirements of a preliminary study as opposed to a full scale study. It should be made clear in the language of the budget bill as to the depth of the study being fielded.
People play fast and loose with that term.
What is nationally significant to one person may have little or no significance to another. Unfortunately, other than fine tuning, I don't see any logical way around coming to some conclusions about the national significance of an area being considered for Park Service inclusion. The obvious examples are easy. No one would argue about the national merits of a Grand Canyon, a Yellowstone, the Statue of Liberty, Independence Hall, Ellis Island or similar national treasures. But finding consensus on a number of other park units in the system would be much more difficult.