>> \baselineskip=0pt means that the height and depth of a text line is 
>> equal to the maximal height and depth of the characters in that line. 
>> See the attached two screenshots that demonstrate the difference.
>
> That's what Emacs does as well, when line-spacing is nil, but Augusto 
> says they are different.
>

Unless I'm missing something, that's not what Emacs does, no.  I attach 
two screenshots of the exact same text with the exact same font, one with 
Emacs (setq line-spacing nil) and the other with TeX (\baselineskip=0pt). 
The effect is very different.