This is the most informed post here so far. A couple of important points to add.

The problem is 2703(f). It allows a government entity to request a snapshot of a user's account "pending the issuance of a court order or other process", meaning that 180 days later, the government can just issue a subpoena - they get their foot in the door and then come back later with the lowered bar, and only need a subpoena. Same effect as not having warrant protection. http://www.law.cornell.edu/uscode/text/18/2703

It's important to point out that the 90-day notification can be renewed indefinitely, pursuant to 2703(f)(2), which is probably a breach of what the 4th Amendment intended.