Besides the obvious qualities we all look for in speakers such as great frequency response that is both extended but more importantly smooooth and balanced top to bottom, nice tonality that is pleasing to the ear and a non fatiguing sonic signature, there are also other things to consider. I've long ago stopped thinking of many higher end speakers as a "single element". It is in fact a "system" that is comprised of your room, acoustics in your room, partnering gear, cables and to a lesser degree for a "great speaker", choice of music you like.
Having heard many of the same higher end speakers in different rooms with different levels of acoustic treatments, partnering gear and cables, one can walk away with quite a different impression every time.
As long as the speaker doesn't have major flaws, there is always a strategy of how to optimize their performance in the room and extract more as well as tilting the system balance closer to your personal preference.
Of course for
all of the above to be true, a speaker has to have great dynamic range capabilities, able to convey dynamic contrast and shading in the recorded music effortlessly, have as little coloration as possible from drivers and cabinet, recreate spatial information present on the recording in the form of imaging/soundstage to make your room disappear and create the illusion of the listener being part of the musical event that was recorded in another place at another time... Assuming the manufacturer did his due diligence and designed a speaker that has good to great response that is free from audible coloration, hardness and brittle response from the tweeter, etc.. then tonal balance and tweaking the speaker further to your liking is easy enough to do if one knows what to look for in partnering gear.
Having owned many speakers before and having heard even more, Wilson, among a few others can and does meet all of those parameters effortlessly and that is why I have become such a fan.
Doing a little planning and research has finally paid off in my own system. I couldn't be happier with my Wilson/Lamm pairing.
Having said that, I am sure I could come up with at least another dozen combos that would make me very happy as well.
Keeping in mind the "system" approach, it is quite possible to take a modern higher end speaker that meets the above criteria and make it sound better than any other speaker that has not been system optimized and visa versa. The rest is personal preference, budget, aesthetics, etc...