There is a maximum weight that an aircraft can take off. If you're over the maximum weight, the plane cannot aerodynamically support the weight and cannot take off. Remember the US Airways Express flight that crashed on takeoff in CLT. That flight was believed to be overweight.
<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by grouse:
Why not burn the fuel off in the air during a holding pattern before landing if necessary? What if that were to get stuck in a pattern anyway and they were to wish they had kept that extra fuel?</font>