Bad light: The light is usually coming from the side leaving one side of the face high lighted and the other side of the face is rather in the dark with a very strong contrast on the face. It's where they put the light and with what intensity and focus. Often the background is very dark too. Maybe my TV is badly adjusted, although I find it more annoying with TNG than other shows on DVD/BD.
Are you referring to the low-key lighting in this shot here:
http://tng.trekcore.com/hd/albums/1x01/farpoint_hd_056.jpg
If so, the look is intentional -- that's how it was lit by the cinematographer in 1987. Compare what you're seeing to TrekCore's TNG screencaps. You're HDTV should be calibrated to look very much like what you see there. If you have a Blu-ray with a THX optimizer, you could calibrate your set using that and it should get you pretty close to where you need to be brightness and contrast-wise.