Terror pilots not experienced enough to deal with the severe weather or complex aircraft systems?
Seems unlikely. If you recall, on 9/11, the terrorists purposefully used cross-country flights to attack targets relatively close to the airports where the flights took off. This was so that the planes would be full of fuel and thus more destructive upon impact. On the other hand, in this case, by the time the plane got to Europe, it would have been relatively low on fuel.
It just doesn’t make sense that terrorists would take over a transatlantic flight so early into the flight. A bomb is certainly a possibility, but hijacking seems unlikely.