Part of me hope this means the plane didn't crash and the pax were somewhere, held against their will but otherwise, unharmed.
If WSJ reporting is accurate, the engines only worked about 5 hours instead of the maximum 7.5 hours it could have flown, it would seem to me the likelihood the plane crashed due to fuel exhaustion is low even if the plane ended up flying at lower altitude than normal. So the 5 hours marks would pretty much rule out crash due to fuel exhaustion. If fuel exhaustion is not responsible for the disappearance and the plane continued to fly for an additional 4 hours after the initial disappearance and presumably in a determined direction - no distress call and no automated warning of anykind - then it would seem the most logical conclusion is some kind of human factor, possibly deliberate. Whether it's terrorism, hijacking or whatever, it is likely something deliberate and human as opposed to mechanical or electrical.