How, within a simple few seconds, do you get a significant player base frothing at the mouth? You get a Valve executive to teasingly point at the number '3' in an elevator, before choosing another level in the building.
Naturally this might be thought of as a Half-Life 3 trolling, but there's a real significant chance this might be for a Left 4 Dead 3 reveal. It's got an 'L', a '3' and a '4' in the clip.
As for the 'Dead' part? Well, many Half-Life fans will be wishing Marc Laidlaw were 'dead' right about now. Besides, we all know elevators are a classic tool of tension in zombie flicks.
Of course if you really want to stretch it - elevators are kind of like primitive portals maybe?