There's also the fact that the plane was on the ground in thousands of pieces scattered over a wide area. The only way that can happen is if it hits the ground at close to supersonic speed (unlikely from low altitude just after takeoff) or if it broke up in midair (unlikely to happen just because an engine failed). As soon as I saw that crash site, and given whats been going on over there of late, my immediate thought was missile strike.
Also fishy was the announcement (by the Ukrainians, but which must have come from the Iranians) claiming it was "definitely" engine failure that was made within a few hours of the crash, before anyone had managed to recover the black boxes much less examine them.
Oh yeah, and the statement from Iran that the plane turned around and headed back to the airport yet the crew didn't put out any kind of distress call also stretches credulity. Aside from the fact they'd need to radio in to arrange an emergency landing, I find it hard to accept they had time to do one but not the other.