The basic issues that relate to NAFTA have to do with the extent to which the various countries will open their markets and the extent to which they will continue to keep them closed on various pretexts.
For example, many Americans want to keep our market relatively closed to Mexican products. They feel that Mexican workers are treated poorly and Mexican companies do not have to follow US health, safety, and environmental standards. This, they feel, gives Mexican firms an unfair advantage. Therefore, they try to keep Mexican goods out by making rules about things like health and safety issues. These are not tariffs or trade barriers so they do not necessarily violate the NAFTA agreement. The issue is whether these health, safety, and environmental concerns are legitimate or whether they are simply made up as a way to keep our markets closed. An example of this can be seen here in the issue over Mexican trucks being allowed into the United States.