It's not so much the "negotiating" with terrorists part that gets me. In the past, based upon discussions I've had with military intel members who were stationed in Afghanistan, I've even advocated talks with the Taliban.
My problem is that Congress said "The President must inform us 30 days in advance of any prisoners leaving Gitmo." Not, "if he feels like it" or "if he doesn't mid terribly". He knew this law, he signed it. And, he violated it.