Don't get? Most Romantic era classical (with very few exceptiions), although I love ancient, early, early baroque, baroque, post modern, post-tonal, etc. And yes, Mahler

.
Opera? Can't stand it, don't get it, never will. I've been to more performances at the Met than I can count, seen many of the greats, and with the exception of a few select arias, it doesn't work for me.
Beatles? Have to be highly selective and then only in small doses.
Zappa, sure do get him -- a true genius, and absolutely amazing musician, but I can't say I have any of his recordings. Generally don't miss not listening to him.
Grateful Dead: took me a long time to get it, but once I did, and tempered with a bit of selectivity, something I truly enjoy. Lots of crappy performances and recordings out there, but the good ones are wonderful.
Don't get new country or new "formula" radio rock. Whenever I turn on a radio there is just so much that sounds like the song before it, and singers seem to be getting seriously bad.
One artist whose music I like is Ben Harper, but he just can't sing or play guitar to save himself. So, while I enjoy it, I don't get how he even got in the door of the first recording studio.
Rap, what's rap?

Lithuanian marching music?
Polka?
It's easy to get carried away here.
-- Jim