@Barkworm Unfortunately, I can't answer that as freely as I'd like to. I think the majority of any personal disappointment I have with the series is related to:
- The lack of epic 1v1 fights.
- Oda's art becoming more and more scribbly.
There are certainly points where I feel Oda dropped the ball on emotional/story levels.
Ex: "I don't know how Luffy will beat Kaido."
Answer: He'll punch him really hard
But they're not enough to ruin any of the experience. The above two points though, they're heartbreaking.
I would love to go into both of those points in detail but because it turns out they do have reasonable explanations but in both cases they deal with info I know from working on 'that side' so while the series is running, it's not my place to bring them up. The only thing I feel comfortable saying, since it was my own speculation to begin with, is that for Oda to do 1v1 every single arc would be even beyond his superhuman ability as a perfectionist as far as being dedicated to time. It would take a true workaholic on the level of Tezuka to flesh out every arc with those kinds of encounters.
Some people claiming to be looking out for his health are the same people who will moan and groan about lack of fights when they don't realize, their absence is Oda looking out for his family, health, and future.
So with those being, probably, my two harshest critiques of the series (since he added current story-line death back into the realm of possibility) I can't see too much ruining my enjoyment of the series by the end.
There's more to this than I'd like to say but, especially about how my impression of the end could be impacted, but for now, I think this addresses most of what you were wondering!