I am self-taught and never completely comprehended exactly what good left-hand position was when reading about it. It is only now that I am an intermediate level player and playing more complex lines that I have begun to notice the dull pain in the back of the hands after playing. This issue actually just began for me in the past few days and today after reading this thread, I sat down and decided to figure out fretting hand position once and for all (gingerly). This thread, combined with the image below, made it finally click what was meant by thumb symmetry with the middle finger, and I immediately saw how my pain was caused by the bad angle between these fingers.
One thing I am still wondering about is about the amount of force other people use when fretting. I read some people here talking about being able to press the strings down with out their thumb on the back of the neck. If I try to do this, it seems to encourage the dull feeling at the back of my hand. Does it make sense that a bit of pinching may be required for people like me, with long, presumably weaker fingers? In the picture I posted above, it looks like there is definitely a bit of pinching going on. When the whole "thumb in line with the second finger" thing finally clicked, it seemed that the little bit of pinching actually felt good, taking the pressure off the back of my hand. ...So what's the deal with pinching? Always bad?