I never understood what the Deadheads were seeing or hearing, but I never took those heavy drugs .  50's jazz, while I respect, is boring to me.  My jazz started in the 70's with Weather Report and Tom Scott.

This is a great thread.

As for Bruce, his first seven or eight albums have a firm place heart that speaks to by blue collar upbringing and my journey into adulthood, but I understand why people don't connect with him.

Same with the Grateful Dead.  It took me quite a while to get around to them and dive in, but American Beauty and Workingman's Dead were the gateway, followed closely by the Barton Hall '77 bootleg and the live albums that followed American Beauty (Grateful Dead and Europe '72).  I still don't get their second and third albums, though, the ones they supposedly mixed while high on acid.

Dylan is another one of those who took me a while to connect with, and I will say that most of my friends don't understand him, either.

One album that's supposed to be great that I've tried (and failed) to appreciate is Radiohead's OK Computer.

At the top of the "I don't get this" heap would have to be Captain Beefheart's Trout Mask Replica.  Safe as Milk is pretty darn good, but this follow-up is too much like that annoying guy in high school who is genuinely trying to be annoying (and succeeding).  There are many artists and critics I respect who revere this album, but I've tried and been left baffled.  I've been told you have to take it in pieces and let it grow on you, but anything that takes that much work to enjoy can be left by the side of the road for something my heart naturally connects with.

There’s no accounting for taste. One man’s wall hanging is the next they’s door mat.

Rammstein and Hüsker Dü aren’t my cup of barf where as others don’t get the seminal “Kind of Blue”

I will never ever ever ever understand Rush. I don't know what annoys me more, them or black metal.  what is the point of barfing into a microphone and recording it?

Miles Davis and "progressive jazz" in general after Kind of Blue/Sketches of Spain.

Too much rock, blues, classical, folk/acoustic, big band - "proven" and new - to enjoy for me to use too much time trying to "get" music that doesn't resonate.  My candle of time is way shorter than it was when I actively sought new music appreciation experiences. YMMV