Timbre/Tone is the very first thing I notice that grabs me. It's what got me into high end audio (and I guess imaging/soundstaging did too).
If a soundsystem doesn't have an attractive tone, I have no reason to sit down in front of it and listen. (Whereas I can listen to any other systems doing other things, e.g. background music, driving in the car, whatever).
Ideally what I hear are voices and instruments sounding similar to real life timbre. The problem is most reproduced sound is homogenized in timbre. Once I hear a pair of speakers play a few tracks, especially ones I'm familiar with, I know how drums, cymbals, trumpet, sax etc are going to sound through those.
Given very few if any systems can truly reproduce the complexity and range of instrumental timbre, what I ask for as compensation in a speaker/system, is a beautiful tone that is at least complimentary. If I'm choosing between a silvery electronic timbre imposed on the sound, or a warm woody organic tone, I'll take the latter. Especially when I hear an acoustic guitar (metal string) I like to hear that woody, sparkling golden harmonics that I associate with most acoustic guitars (including when I play my own). If a speaker can get that right, I know it's going to generally grab me.
Though beautiful tone isn't enough. In order to hold my attention I want a very good "disappearing act" with excellent imaging/soundstaging. That is after all what stereo is all about and why it makes sense to sit between two stereo speakers in the first place. Otherwise I could listen to the music in the back ground from anywhere else.
But tone/soundstaging also go only so far. I need some excitement, so I need some dynamics and physical presence. I started off with Quad ESL 63s and they did beautiful tone, a wonderful disappearing act and soundstaging, but after a while I missed the physical connection to the sound that I got from dynamic speakers.
Though of course it turned out not all dynamic speakers give enough of that. I went through some that had richness, fullness and wide frequency response....but still a bit too "airy" and polite. I wanted more solidity to the sound. Horn speakers tend to do that well, though I never went that way for various reasons.
Now I have speakers that come as close to doing it all, hitting all my buttons, as I've ever had. The imaging is dense and convincing, with a particular sollidity and "thereness" to objects in the soundscape. Instruments just feel like they are there, not see-through. In electronic music, which I love, when a new synth sound appears it just ripples the air, appearing almost like a new physical presence in the air like I could reach out and grab it.
But I also know that no matter what system I have, in time I will be aware of it's imposed voice and I'll start sniffing around elsewhere. That's one reason why these days I tend to keep more than one pairs of speakers around (actually, I have at the moment about 5 types....I need to cut back).
Anyway...sorry for the dissertation. As an audio-geek this kind of stuff gets me going.