What's the most important speaker attribute...and


why?
That ineffable "musicality" is IT for me. The ability to reproduce human voices that sound, well, palpably human and distinct (not all raspy, if male, or sibilant, if female).
Instruments sound like they do "live."
In a nutshell, you don't gush to your friends, "Hey,I keep hearing new things all the time," but rather, " Billie sounded soooo beautiful," or, " the counter tenor blending with the bass---magical!"
If it doesn't sound beautiful, if the instruments and voices aren't naturally seductive---you're not listening to music anymore, no matter how impressive the stage width, depth, or heighth, or how dynamic and uncompressed it is. (My system is currently in large scale flux, from front-end to speakers; I just went to SF EAIIs [from Piega C8LTDs], Pass Labs electrs, Audio Note cdp, but I'm swapping out passive preamps this week [Preeminence & con-john pfr] seeing if they make significant improvements).
128x128jayme
For me, it's not so much a particular spec or trait but instead it's can I tell the difference, on a recording, between a Gibson and a Martin, a Guarneri and a Stradivari, etc. The most reasonably priced speaker that can do this, irrespective of laboratory measurements, is the one I'd buy.
Try listening to Spendor S3/5's in a small room. I have heard very few speakers that can do what these do. Not for thrill seekers, but if you want to just listen to music and relax....
Midrange, if it ain't right nothing can possibly be.

I use the LS3/5a's on the side along with another full range speaker and for the price and size it would be very hard to beat. It is a good speaker to use for reference to evaluate other speakers midrange as well. Sure there may be others which may have a better midrange but one has to dish out a lot.

To each his own!
The most important aspect is that you like the sound of them and can listen to them for long periods. Flat frequency response over as wide a range of frequencies is important also. Look for bass extension to at least 40 Hz. Decent speakers and good electronics make for a system that is a pleasure to listen to. Go out and audition as much as you can with music you own and take notes. Have fun while you do this too!