QUESTION: It would seem that "detail" would vary with what one wants to pay close attention to, no? Or is listening to the pace or the bass or something not in the upper mids and treble not considered 'detail' by most.
I don't 'know' what is considered detail by 'most'. The reason I don't know is there is a sometimes vast chasm between what most say and what most do. A lot talk a good talk but then when you see what they do, oy vey!
Detail is simply the finest smallest most subtly perceptible aspect of a thing.
Let me give you an example from watches as a metaphor for audio. Show a Breitling and a Grand Seiko to the average person, or even the average watch person, the vast majority will say the Breitling is the more detailed of the two. What they mean is there are more sharp edges, more obvious tiny little decorations. A more sophisticated view is the Grand Seiko is the more detailed by far. Details such as the way the case curves into the bracelet, the ineffable look and feel of the zaratsu polishing, the way the inside edge of the date window is that same zaratsu polishing, and more, are all there for the discerning eye. They just don't jump out at you. Most see what jumps out at them. Most see the obvious details. Missing the deeper more captivating ones.
Another one, when you detail a car, detailing is not just polish and wax. It is getting into every little nook and cranny, all the little things so hard to see most will never even notice. Sometimes even when they go looking for them. I have judged concours competitions and seen this first hand. Wheels look good. Really? See there and there? Little bit of grime where the lug nut bolts to the wheel? Shoulda removed the wheel, cleaned everything, then put it together. While you're at it clean the inside of the wheel, the brake calipers and hydraulic lines, inside the wheel well. That kind of detail.
It is all detail. Just in audio some are so incredibly finely detailed they are more felt than heard. So yes of course what one calls detail will vary a lot, not only from person to person but even the same person depending on what they happen to focus on.