I'm biased as I played in bluegrass bands for decades. I would guess that a third of our set lists were instrumentals. And for this genre the instrumentals were expected to have a well defined melody and the soloists were expected to stick to that melody or not venture out too far

That's one aspect of my like of instrumentals.
However, I've also listened to rock and blues since I was a kid and certainly got exposed to jam bands in the 60's. So I'll add, from a personal perspective, that if the instrumental does not have a particularly strong (or perhaps any melody) then what can capture me is sheer virtuosity in the solos, i.e., not hackneyed licks, etc.
Then there is another dimension and that is songs that capture me by the feel from, I guess, a wholistic perspective, e.g., a moody or ethereal feel, etc., but not necessarily a lot of soloing.
Pardon the ramble...it's a bit like "how long is a string?" I'm outa here before I proofread this because I'd likely disagree with it!
Bud