My best guess at this point is terrorism as the cause of the Malaysian air crash.
Terrorism would seem an odd choice for a Malaysian flight traveling between Malaysia and China. But China was the seat of a recent terrorist attack in a train station. Although it was a low-tech one in terms of weapons—knives—it killed a lot of people, and the suspected perpetrators were a separatist group in China who happen to be Muslims. Malaysia, of course, is a predominantly Muslim country, although certainly not one that’s usually a seat of terrorism (although see this).
Facts that argue for terrorism as the most likely or at least a strongly possible cause are: the catastrophic and sudden nature of the catastrophe, and the time and place (while cruising, and over the water—bombs are often timed to go off over water, if possible). The fact that two passengers were traveling on passports stolen in Thailand may be an entirely unrelated glitch but adds to the sense of suspicious doings, and recent reports are that the tickets were purchased together, and there may have been other suspect passengers as well. If this is related to a terrorist attack, it would represent something new in air terrorism; stolen passports are not the usual m.o.. But terrorism keeps morphing in an attempt to keep ahead of security.
There’s also a report that the plane tried to turn back, but it hasn’t been confirmed, and it’s not clear that that would be related to terrorism, anyway. Could indicate some sort of perceived trouble by the pilots, although the fact that they did not contact the ground argues against it. The type of plane involved has been an unusually safe one, the weather was excellent, and the pilot a trusted veteran. The disturbing possibility of purposeful pilot involvement cannot be ruled out, of course, but that’s not my leading theory at the moment.
I find forums such as this one (supposedly composed of professional pilots) to be a fount of speculation that contains interesting technical information on situations such as this one.
My guess is that the plane’s wreckage will be found, the suspect passengers will be identified (50/50 on whether they were involved), and some but not all of the mystery will be solved over a period that may take years. The answer, however, will probably be terrorism.
RIP, all souls on board.
[ADDENDUM: More here.]