Character bloat is what ultimately did it in for me. There were more and more characters that all needed panel time, and he insisted on constantly breaking up the crew so you'd have a half-dozen concurrent lines that were all fighting for time. It started making each arc take years to complete. I finally gave up when, after letting it sit for a year so I could marathon it, the arc I'd originally gotten sick of six months in still wasn't finished a year later. Not only not finished, but didn't appear close to a climax.
I feel like the same thing happened to Oda as happens to some famous novelists: they become 'editor proof.' They become so successful that there is no longer anybody left to tell them stuff like, "Hey, the way you have this story blocked out is going to take forever to tell. You need to consolidate plotlines to keep up a snappy pace." Or maybe, "That character doesn't really need an entire chapter devoted to their backstory. You could still manage an evocative vignette with a couple wordless panels."