I think the videos you saw are more about the spring on the hinge that would keep the keyboard open or closed. The spring on the hinge is designed to be loose or weak so as to make opening and closing the keyboard easier and without the hard snap in place when you move the keyboard past half way. HTC has said it was designed to be loose. The drawback is that if you hold half of the phone, gravity can cause the other half to swing partially open and close. I think this is what you see in those videos.
Of the videos I have seen, the person holding the phone deliberately holds it in a way that can demonstrate the loose spring on the hinge. Most of those positions they hold the phone in are ones which no one would normally hold any phone.
I've been using this phone for three weeks now. When the keyboard is closed, there is no possible way that it would open by itself if I hold it normally. When you grip the phone, you would grip the sides in a way that you are gripping both the screen half and the keyboard half. If you are holding the phone on the sides, your finger will naturally grip both halves, keeping it together. There is also some tension on the spring that holds it close.
With the keyboard open, I'm usually sitting up so the weight of the screen keeps it closed. The only position where the weight of the screen would force it to close is if I am lying on my back looking up at the screen and I am holding the keyboard. If I do decide to use the phone in this position, which is very uncommon, I can always prop open the screen with my index fingers while my other fingers are on the back of the phone and my thumbs are free to type.
I think many people expect the spring to be tighter than it is. That is the main source of the criticism. The loose hinge does not, however, mean that it wears out faster. I cannot really comment on the longevity of the hinge mechanism as I only used it for three weeks. I haven't notice any degradation of the quality of the hinge in these three short weeks.
In comparison, the Nokia N97 has a similar mechanism for its keyboard. Unlike the Desire Z, the N97 hinge mechanism has a rather tight spring. Gravity will not force it to open and close because of the tightness of the spring. The disadvantage of the tight spring is that it makes the mechanism more difficult to open and close. Also, after you open or close it more than half way, the spring causes it to snap hard in place. Some people do not like this property of the tight hinge. I have seen people criticise the N97 for having a spring that ws too tight which is the opposite of the criticism of the Desire Z.
It all depends on your particular preference. I have used both the N97 and Desire Z. I have not problem with either hinge mechanism. I don't think that people should have anything to worry about as long as they grip the phone normally.
thanks for the time you put into writing that essay aswell! appreciate it 
