MTOW/MZFW and actual lift potential.
Since a new 777-200LR is about $300 million, it's a lot cheaper to steal one for purposes of a terror attack than to go and buy one.
Depending on how much fuel you have to load to reach your target, these things have an enormous lift capacity. On 9/11, it was not the impact of the aircraft, but rather the tens of thousands of gallons of fuel that did the most damage.
If you remove all the freight and reload with with explosives and just enough fuel to reach your destination, it occurs to me that you would have one heck of a flying bomb.
With careful maintenance, aircraft can be flown repeatedly at the maximum payload limits. But what if you didn't care about being able to fly the aircraft tomorrow?
If you could find a long enough strip with a headwind and no nearby obstructions, other factors being equal, how much could a plane like this lift off and eventually climb with?
I would never have asked this question immediately after the disappearance, but given what we know now, it doesn't seem so crazy.