If I'm correct the big fear is the President can sign a treaty and the United States would be bound by the laws of said treaty until it is brought before the Senate for ratification, pass of fail. The only problem is with Harry Reid as leader in the Senate, it would never be brought to the floor for a vote, so we would be bound by the treaty for however long it would take to bring it to a vote. Also, the big deal with the treaty is this. The UN says it's to stop international arms dealing. That's cool and all, except they would require signatory nations to maintain a database of all firearms inside their borders to control the flow of guns outside the country. Lots of bills are seemingly harmless, but can pose a real danger if you look into them ( See Patriot Act). The last thing many of us want is the UN, let alone our own government, have an inventory of addresses and firearms.
1st bolding: Perhaps one of our Constitutional scholars here could correct me, but my understand is that the only part of the treaty the United States would be bound to would be that we could not take actions that would invalidate the treaty for other nations, but that the President's actions would be EXTREMELY limited until it was ratified.
2nd bolding: Not what I've read in the least - only read that internationally traded arms are covered by this, so if an American buys a gun made internationally then yes they would be in the database, but if they buy a domestic gun they would not. I see no problem with this at all.
Finally, how was the Patriot Act at all seemingly harmless? It was terrifying from the very start... Do you mean a lot of people didn't realize how scary it was? Those people just didn't look into it at all.