People have posted a lot of "it could have been" or "must have been" possibilities, but the one that I haven't seen mentioned (and maybe I missed it) is the possibility of pilot/crew suicide as in Egyptair 990. No one really wants to think about that and the probability of it being the cause is very, very remote, but it is still, a possibility.
I can't see any similarity with EA 990 or LAM 470. Both aircraft were tracked on radar when losing altitude. Not with MAS 370. Apparently the last secondary return available indicated an altitude of 38,000 feet, then nothing. This fact, if confirmed, is certainly disturbing and raises lots of questions, but more or less excludes any similarity with the aforementioned crash cases.