I thought the exact same thing. Then I downloaded
Brightness Level. At different times with varying degrees of ambient light, I set the brightness to "Automatic". Each time I manually adjusted the slider to duplicate the brightness level Automatic provided, switching back and forth between manual and automatic until I matched the levels. Each time the matching manual level was a different percent. This proved my theory. The automatic brightness setting is working. However, it adjusts the screen brightness so seamlessly, you don't really notice it's working. This is exactly how it's intended to work. It should look exactly the same under varying light conditions. If you think about it, it has to produce different light outputs in order to accomplish this, since your pupil continuously dilates and constricts.
As for the soft buttons not illuminating until near total darkness, I agree. The sensitivity level needs to be changed via a firmware upgrade. I've already contacted HTC Support about this. It would help if they received multiple complaints. If I'm the only one, I'm sure they'll just look at me as being too particular. You can find your phone's serial no. without removing the battery. Just go to Settings/About/Phone Identity. You'll need this to fill out the form at the below link.
HTC Mobile Phones - Customer Service - E-mail Support
RH