It slows down gastric emptying as one of the primary ways it helps people feel full longer, so there a slew of possible GI side effects (nausea, constipation, diarrhea). In general they are manageable due to being able to adjust dosages to where patients still lose significant weight and minimize the impact of those side effects.
Possibly none. These drugs are bringing our appetite under control in an environmental that is full of readily available, appetizing, easy to digest, high-calorie foods.
Well... when we were like cave men and calories were hard to get, hunger pangs make more sense from an evolutionary standpoint.
But now, through farming technologies, food has never been more abundant. Yes we've saved people from dying of famine, but then the corn producers grew so much more corn that were beyond America's daily dietary need, they then could use much of it for HFCS, say. Or you know, Taco Bell's fourthmeal.