I suspect that investigators will already have a fairly good idea what — so sadly — has happened to Malaysian flight 370. Key to this will be the ACARS system. ACARS is an automated reporting system that automatically reports messages back to the operating airline during flight. They range from serious faults to minor maintenance warnings, as well as periodic updates on location.
Key to working out what happened will be whether MH 370 sent any unusual ACARS transmissions, or indeed any ACARS transmission at all. In the case of AF 447, although no pilot-originated communications were received by any land station, Air France still received a number of ACARS messages. These gave enough information for informed commentators to work out roughly what happened, even before recovery of the cockpit voice recorders: a pitot fault, followed by loss of automatic pilot and disagreement between the onboard computers; then a descent rate warning. Then nothing.
Indeed, commentators (many of whom were professional pilots) posting on pprune.org were discussing a ‘deep stall’ even before the wreckage site and CVRs were found (e.g. here). The scenario they posited was verified when the CVRs were listened to: it started with a pitot failure, which was correctly identified as being caused by icing from the large storm system they were entering (a common feature in the area in which they were flying, the ITCZ). Loss of pitots causes a loss of airspeed indication and hence of automatic controls (a scenario known from crashes involving BirgenAir and AeroPeru) and, in the case of Airbus aircraft, a loss of computer protections . In that state, the plane is flyable but is sensitive to pilot input. Control is harder at higher altitude; AF 447 was at cruise altitude, which (if you didn’t already know) is high.
In AF 447’s case, pilot error stalled the plane irrecoverably. The co-pilot pulled back on his stick virtually the entire time, which is precisely the wrong thing to do, and failed to identify he had stalled the plane even though the on-board stall warning — when it was operative; the accident revealed a poor design condition that deactivated and reactivated it in a confusing manner — mentioned the word ‘stall’ about 75 times. (The root cause was likely due to a lack of training and experience at cruise altitude and a concomitant lack of piloting knowledge due to institutional over-reliance on automated systems: it is expensive to let pilots manually fly at cruise, and a crew used to having the computer vet all command inputs loses, or never gains, the ability to fly manually when the computer stops working properly.) If the co-pilot had not touched the stick, likely nothing would have happened; the pitots recovered when the ice melted, and had he maintained attitude, altitude and power, the plane would have continued in level flight, subject to some turbulence.
Worse, he did not even mention what he was doing until near the end (below 10,000 feet, falling fast, when he finally said “Mais je suis à fond à cabrer depuis tout à l’heure!” [But I’ve had the stick all the way back the whole time!] and the captain, who was by then back in the cockpit, face-palmed and said “Non, non, non… Ne remonte pas… non, non.” [No, no, no… Don’t climb… no, no.].) The other co-pilot, David Robert, started corrective action, pushing the nose down, and the plane started to recover; but it was too late, as there was insufficient remaining altitude. In any event, Bonin countermanded this by pulling back on the stick again when the ground proximity warning sounded at 2,000 feet.
In the case of MH 370, one possible situation is that no abnormal ACARS messages were received. That would suggest sudden catastrophic failure that prevented even a single message being sent. (Note that in the case of QF 32, many messages were sent right from the instant the engine failure occurred, so, if MH 320 sent no messages, then the loss of transmission ability must have been instantaneous.) That would in turn suggest catastrophic structural failure either endogenous to the airframe (e.g. as with China Airlines 611) or from an explosion (as with TWA 800).
The other situation is that some unusual ACARS messages were received. If so, those would be being carefully scrutinised right now…