I second what LongTimeInCX says about Captain's competence levels with regards late interventions. It's not easy, especially in the last 100ft, to intervene either verbally or physically. Will a firmly stated warning help or possibly overload a less able pilot?
I go back to the very basic cliches: a good landing usually follows a well-flown stable approach. In my experience as PM, I can normally tell quite soon after the PF disengages the automatics how they will perform the actual landing. But this impression is not foolproof and I've been caught out when a nicely flown approach didn't lead to a timely flare and I found myself gasping 'Flare!' at the last moment.
It's tricky. When an aircraft is descending at 12ft a second until the flare, you simply don't have much time and any intervention has to be carefully managed. I've not yet had to take controls on an approach, but I can only trust my instincts as a pilot to do so if the time arises when it doesn't feel right.
On the point of the FL40 landing flare, in my experience many inexperienced pilots do not flare correctly and land too flat. This is despite a commonly held understanding that the aircraft's nose is pitched lower when using FL40 as opposed to FL30. I regularly find pilots flaring to a point with very little positive pitch and then, when they find no immediate touchdown, they relax the flare. The subsequent landing is usually firm because of a loss of airspeed. Worse, it's very flat and nose wheel touchdown is very soon after the MLG meaning that you most certainly are not flying the nose wheel onto the runway, as you should.
PS: Most of you will remember this event:
http://www.flightglobal.com/news/art...d-a320-320070/