Disappointing stuff (that has little to do with the current conversation going on)? I'd say, mainly, side characters that I wanted to shine and yet were, in the end, pretty much just. . .there.
The biggest offender is the Fishman Island arc. Shirley, anyone? She had so much potential, for what? Aladdin? Are you there? Anyone knows why Den was introduced? In comparison, characters like Neptune or the princes were given top notch treatment (in fact, I'm quite happy of how Oda treated Fukaboshi and co, even if off-panneling their fight with Hodi's gang wasn't the smartest of decisions, seeing how their fight would've probably been much much much more interesting than the extremely one-sided beatings the SH's gave them).
I get that all of them will probably reappear again in the future (that's been so heavily hinted that I don't think doubting it is a possibility), but still, in a 40+ chapter arc, not giving your secondary characters even a small chance to. . .do something, that's just so unlike Oda.
During Impel Down, Saldeath looked like he might be doing something, but in the end he became extremely irrelevant. Domino is an even more of an extreme case. Why did Oda even bother giving her a name and an intro box? Was she there for, what, 4 pages?? I could say that Sadi could've done more, but that's because her fight with Ivankov got off-panneled, otherwise she got some decent pannel time and we at least got to see her quirks.
I have a small grip related to the above, now that we are talking about Impel Down; I loved the arc, but the second half is SO RUSHED. It's still good stuff, and I get the reason behind the frantic pace of the last chapters: Oda wanted to give us a real taste of how Luffy was running out of time; the same happened during Marineford, where we didn't get to see almost any fight at all, because the main event going on was Luffy trying to save Ace against all odds and against time.
But, seriously, would it have hurt to give that arc 4 extra chapters? Just that! Hannyabal deserved a better treatment during his fight, Sadi deserved to show off a bit, maybe show Saldeath doing something, show in full detail how everyone got out of Level 6 (btw, why is the level where the worst of the worst is gathered the most normal looking, pleasant one? No extreme conditions, no mass murdering animals, no specially strong guards. . .nothing?), etc. . .
I'd say the many many many plot conveniences and leaps of logic that were there during the Marineford arc, some of them pretty hilarious (Kizaru's sniping abilities, "don't move a muscle, Kizaru! or else. . .oh, well, he moved and now is throwing a neverending barrage of hyper destructive light rays, damn him. . .I'll just stand here looking at him. Manly", or Sengoku's super badass sleeve roll up that in the end was just a tease, cause he kept standing there shouting orders like nothing happened, etc. . . .), but I still think that arc has so, so, so much merit in how it conveied a freaking BATTLE and all the chaos surrouding it, and how Oda filled each and every pannel to the brim with dozens of unique looking characters duking it out in a massive scale, that I just can't bring myself to dislike it. In fact, I love it. Even if it's, quite possibly, the most flawed arc in the entire series.