Hmm, two off the top of my head:
re: Game of Thrones - I had started hearing about this series around 2011/12 and wound up getting a box set with the first 4 books for cheap from Costco to give a shot. Sounded pretty cool: huge fantasy epic, lots of interesting characters, dragons?? But... I couldn't make it beyond chapter 10 or so of the first book. I tried like 3 different times separated by a year or more each. Something about his writing style and/or pacing just did not click with me, like at all. Everything just moves at a glacial pace and it's bouncing between too many characters before letting you get acquainted with any of them first, and blegh. I tried the show out, watching 1/2 or so of the first episode and gotta say, I do think I would like that better just because it moves at a faster pace. But I can't be bothered at this point.
Attack on Titan - This one's really petty, but back when this series was first starting to air its anime and it was all the rage, one of my shonen loving friends insisted I had to watch it. I'm also a huge shonen fan, so a few days later I sat down to watch the first episode and... the art style just doesn't jive with me for some reason. I couldn't continue watching past Ep. 1 because I didn't like how it looked :X