One danger I see with a lot of eating plans is the overall relationship that a person develops with food. It almost seems like some plans make food out to be evil - something that needs to be controlled and guarded against.
In the long-run, you would want a person to have a healthy attitude to eating, not a neurotic one. I think this can happen on both sides of the eating equation, whether a person is trying to diet and lose weight or eat more and gain weight. Too much command-and-control spells trouble in the long run.
For example, the other night I literally "forgot" to eat dinner. I was busy and apparently not hungry, and I didn't realize until I woke up hungry the next morning that I hadn't eaten dinner. But to me, all this was no big deal.
It works the other way too. Some people count every calorie and try to keep their eating so tight that they are setting themselves up for trouble. It's like they put themselves in a pressure cooker, and eventually it will blow.
Generally, I just try to eat Paleo foods and eat when hungry. But if occassionally I want to eat a not-so-healthy food, then i just go ahead and eat that as well. I think any negative effects from the food are ultimately outweighed by the fact that I don't make a big deal out of it. If forbidden fruit isn't really forbidden, then you don't desire it as much. And so a long-run strategy of eating freedom will probably yield better results than a controlling approach.