It is dangerous because it is too heavy withstand the stresses of a landing, resulting in the aircraft breaking apart. This is the reason airliners dump fuel or circle an airport to burn fuel before an emergency landing.

Besides the weight of the fuel, if it is FULL, then there is no "SPLASH" space for the fuel ... if a full plane hits the ground, the fuel will be bounced around and it may spill some fuel, and with the heat of brakes, it may catch fire ...

It isn't really that "dangerous" for a "fully fueled" aircraft to land. I fly for an airline. That is just played up a bit too much on Fox News in the rare emergency they cover to garner news ratings. If it weren't "dangerous" how would they get ratings? It is of a concern when: 1) the aircraft is experiencing some sort of emergency where having extra fuel on board could be hazardous if the integrity of the fuselage or fuel cell is broken on landing (fuel doesn't "bounce" out and hit hot brakes!). It is always of concern in such situations where the condition of the gear is in question, as the aircraft could end up sliding on its belly, rupturing the fuel tanks in the wings and spilling Jet A all over creating a fire hazard. 2) the aircraft is above its maximum allowable landing weight (the aircrafts weight has not been reduced to that weight as fuel has not been burned off as it would during the course of a normal flight). An overweight landing could stress the landing gear and airframe. However an overweight landing alone certainly would NOT result "in the aircraft breaking apart" (as posted earlier) unless the plane actually crashed. Many, many times flights have made overweight landings necessitated by a sudden need to return to the field(like a medical emergency)and the passengers never even know about it. The maintenance team will inspect the airplane for stress damage and either repair any problems or return it to service. The latter usually happens as they are built to withstand far more than they are limited to structurally. It should be noted that not only do airliners circle to burn off fuel for the factors mentioned above but doing so also buys the pilots TIME. Valuable time spent burning fuel also gives the crew extra time to troubleshoot their problem and talk to the company dispatcher and maintenance on the radio.