Adam Serwer does a nice job of summarizing the National Defense Authorization Act without getting apoplectic about it. To summarize his summary:
1) The bill codifies indefinite detention.
2) The bill does not permit indefinite detention of U.S. citizens.
3) The bill is dangerous.
My view of the bill is closely in line with Serwer's.
Here's what can be done at this point.
Contact your senator and ask them to oppose a veto override. Why? Because even if the president were to veto the bill, there are enough Senate votes to override the veto and automatically pass the bill (a presidential veto would be merely symbolic).
The other thing we can work on is to suggest a signing statements for the president to add. If the president declares an end to "hostilities" in a signing statement, then the indefinite detention language becomes moot.