The NDAA allows for the President to detain Americans indefinitely, without a trial. These people may be completely innocent, and simply mistaken for terrorists by a false association...without a fair trial, who could tell? In this sense, America loses its title of "land of the free".
The law can also be abused by future presidents, who wish to silence journalists or opposing political party members. That is not America. The 6th Amendment to the Constitution provides all American citizens the right to a fair trial.
For some reason, the Obama Administration has vehemently fought in favor of this law in courts, where it was recently deemed unconstitutional (see truthdig links). WHY????
For more information, read:
http://www.aclu.org/blog/tag/ndaa
http://www.truthdig.com/tag/nda