OK, here are the facts. None of these are disputed claims (e.g. military radar detecting the plane flying west, Chinese satellite images of the plane, oil rig worker account of a burning plane, etc.). The report of the ACARS transmission continuing for four hours after the disappearance is still disputed but may end up being correct… we’ll have to await more news.
1) MH370 took off from Kuala Lumpur International Airport at 16:41 UTC 07 March, or 00:41 local time 08 March.
2) ATC lost contact with MH370 around 17:22 UTC, and the plane dropped out of radar. The last known position is 6°55′15″N 103°34′43″E.
3) At that time, the plane was at cruising altitude at 35000 m traveling at 6/471 kt.
4) After almost six days, no reliable eyewitness accounts of a plane crashing/blowing up have surfaced.
5) After almost six days of searching in the Gulf of Thailand, the Straits of Malacca, and the Andaman Sea, no pieces of wreckage have been found.
6) Two people boarded the aircraft with stolen passports; authorities have confirmed these are not terrorists.
7) The captain had 18,000 hours of flight experience; the copilot 2,700.
8) The weather was clear at the time of the disappearance.
What we can infer directly from the facts.
-It has been six days and absolutely nothing has shown up. This either means (1) the debris field is extremely concentrated, or (2) the authorities are looking in the wrong areas. 777 seats double as floatation devices in an emergency so it’s highly unlikely that all parts of the 300-ton plane sank.
-No witnesses means (1) there probably was not some major explosion, (2) the plane, if it did crash, did not crash near a populated region.
No explosion => probably not a bomb or fuel tank explosion.
Concentrated debris field => plane was intact if/when it crashed. So not an in-flight breakup caused by metal fatigue, etc.
Authorities are looking in the wrong areas => deviation from flight path => autopilot was turned off.
From these facts the following theories lend the most credence:
-Complete loss of electrical power in the plane => lose flight instruments, autopilot, radio, transponder (which sends signals back to the radar). The pilots, in a panic, become spatially disoriented, deviate from the flight path, and fly the plane into the ocean and/or a remote jungle.
-Terrorists take over the plane, turn off the transponders and radio communication, fly the plane to a remote area. Alternatively, they aim for a destination but run out of fuel before getting there.
-Pilot goes into suicide mode and flies the plane into the ocean somewhere.
-Depressurization of the aircraft. This would explain why it happened at cruising altitude but doesn’t explain the dropping off from radar or the deviation from the flight path. May have happened in conjunction with another event.
Less likely possibilities
-Mid-air breakup (due to metal fatigue, etc.). Again doesn’t seem likely given lack of debris sightings. Could have happened after a deviation from course, but what could explain that?
-Autopilot accidentally switched off, followed by panic and flying plane into ocean. Photos have surfaced of the pilot bringing in “guests” to the cockpit in previous flights… could this have happened and the guests did something dumb? This has precedence (see Aeroflot Flight 593). No reason why this would turn radar and radio communication off.
-Bomb or other explosion. Elaborated on earlier.
-In-flight fire ala SwissAir 111, Valujet 593 et. al. Wouldn’t the pilots have reported a Mayday? It is possible a fire destroyed the electronics so fast that the pilots didn’t have time to react.