Barring single driver speakers I think the ability to marry up a range of different drivers that complement each other is what really makes it.
That seems obvious I know but Let me explain.
Consider a simple 2-way design like the emotion - looks easy enough on paper if one is able to integrate the two drivers' SPL curves and chose an appropriate crossover point but when one allocates so much work to a tweeter which dips well into the midrange the effects of baffle diffraction are significant and in effect will require a complicated filter (butterworth) to counter this and there is a loss in sensitivity to a degree as well as a result. One can take away some of the "complication" in the filter by increasing the baffle size and width , hence the wide box.
This allows us to retain some of the sensitivity as well.
This is just to give you an insight into one way of approaching a problem - the designer has a few other options but with considerations of design, cost etc
Then there is design in terms of closed Vs ported etc, this not only has effects in the tightness of bass but stability of extension as the frequency extends into the all important listening range( 500 hz and above for the midbass driver).
Tightness of bass is not objectively apparent on the spec sheets so qualities like these cannot be accounted for on the CAD drawing .
Then all this has to integrate in the average room , recreate the characteristics with most amplifiers, be acceptable to most ears and sound good with most types of music , be affordable for what it can do, be of a size that is acceptable, not too heavy, and .................. this goes on.
After all this and after doing everything by the book not infrequenty one still find that there is a little something missing , the special experience. This sometimes happens by accident and a special being is born. I have experienced this in many walks of life and am sure others have as well.
I do believe that a clever patient designer can improve on the Lumleys for example, for not much of an outlay .
All IMO Ofcourse - I am not a loudspeaker designer by trade but as a hobbyist I understand the skill and difficulty involved in producing a quality speaker, so when you look at a loudspeaker it is not simply a box with a couple of drivers and some electronic components soldered on based on the findings of a software program. The truly great ones are built with far greater care and attention to detail.