>> completion-first-difference is at the very least a misnomer for
>> other types of completion, because with flex there can be infinitely 
>> many "first" differences. 
> No, "first difference" is always the character to be typed after
> point.  At least for the vastly important case that point is at the
> end of what you typed, i.e. you don't move point back after typing
> something.

But for the 'flex' case (among others more obscure) that first character
"to be typed" -- presumably to narrow down the set further -- might be
any character following 'point', for each completion, not just the one
in the 'point+1' position.  If you use "flex" for a little while, you'll
soon see that this face (which, by default in Emacs, is the only "bold"
one of the two) is quite useless for 'flex'.

Finally, I believe other UI's that have this kind of flex search (I
think they are not rare at all nowadays, not just in editors) also use a
prominent (often bold) face, to mark the common part.  But, like Stefan,
I would like casual users of those other UI's to present the specifics.

João

[1]: Apparently an exclusively portuguese saying.