2hats
Dust.
This pilot has what looks like one of the most plausible explanations I've seen: A tire fire with incapacitating smoke, followed by an attempt to get to Langkawi before it was too late.
http://www.wired.com/autopia/2014/03/mh370-electrical-fire/
All planes that have ever caught fire and continued flying have crashed in under twenty minutes or so, as I recall. There's a rule of thumb - in the case of a fire you have 15 minutes to get on the ground. There'd be a big debris field not far from the Malaysian coast and SBIRS and possibly CTBTO infrasound would have noticed it. The final plunge would have been within range of the primary radar.
Besides, they would have picked WMKC or WMKN instead; they'd have had to overfly WMKC to get to WMKL and WMKN is much nearer anyway. Also doesn't explain the lack of mayday/comms traffic, or observed data (the tyres are instrumented to warn of fire). Recall that BAW38 got a mayday out in the 30 seconds they had, AWE1549 had 4 minutes and managed it; both crews clearly had plenty else to do as well.