I've been following this story too. In my professional life I'm a supervisor at a government media monitoring unit so the news is kind of my thing. This is a really strange case. Solid information has been really slow to trickle out. There's still not really enough data to make a solid prediction. Here's my take on things:
The aircraft's transponder was probably deliberately switched off. I'm no airline pilot but I guess that can be done from the cockpit. The system that sends engineering data back to home base probably can't be switched off from the cockpit (maybe from the avionics bay below the cockpit?) and whoever had control of the aircraft either didn't know about that system, didn't know how to turn it off or didn't want to leave the cockpit to disable it.
The aircraft's movements after the transponder was switched off seem to have been deliberate. It's looking like someone initiated a radical course change and then flew the aircraft nap of earth on an entirely new heading (or as close to nap of earth as you effectively can fly a commercial passenger jet). That sort of action suggests that someone wanted to land the plane intact and undetected. Why would you bother to do all that if you were just going to crash the plane on purpose? Makes no sense.
There has been some suggestion that the engineering data updates back to home base continued past the time that the aircraft would have run out of fuel if still in flight. Logic would suggest that this means the aircraft was already on the ground somewhere and the APU was on. The APU could continue to run for hours longer than the main engines.
But then the waters get muddied even further by stories like this:
Oil rig worker says he saw Malaysia Airlines Flight MH370 burst into flames
Conclusions: It either blew up in flight (terrorist attack or catastrophic malfunction, who knows?) and hasn't been found because there's a whole lot of empty ocean out there, or it was hijacked by a person/persons unknown and was landed somewhere, probably in SW Asia. If it was all done by a terrorist organisation, why no claims of responsibility? Why no intel suggesting an op was being prepared or was in play? I'm not big on conspiracy theories at all, but this case is a really strange one. I'm almost prepared to believe it was a government or corporate black op. That would explain why no-one has said anything about why it was done. Unfortunately, if that is the case, the passengers and crew may end up dead anyway, to tie up loose ends.
Please, I welcome the above thoughts being critiqued. What have I missed?
Edit: Just had a look through the links others have posted. Some really interesting theories there too.