I would probably have to say the show started to go down hill after it switched art styles and switched to HD. The original art style might've been simple, but it stayed Consistent! The newer one is too much of a mess with a melting pot of quality. Also the pacing during Enies Lobby was the worst the series has ever had, and I don't think its going to recover.
This actually has been discussed pretty heavily in the Study of Animation topic. My personal take on it was that "something" cinematic was lost when they went through the animation shift (which, BTW, happened during the G8 arc) but what really affected the series was when the original director left during Enies Lobby where it seems the pacing of the series shifted. I recently rewatched a good chunk of the Water Seven and I felt the pacing of the opening episodes felt consistent with what the series has given us since episode 1 but once Enies Lobby hits…BAM! Another thing to keep in mind is that a lot of the older animators retired from One Piece during Enies Lobby including Hideaki Maniwa and Noboru Koizumi (who was the original character designer for the series). But they also paved the way for Naoki Tate (you know, the Robin prison episodes guy) and a few others. As far as the present day style is concerned I think Toei is trying to update the style to a more fluid and expressive style but the problem is Toei is not putting enough money into it to do it consistently so they mostly use it for key fights and the like.
I wished Toei would've pulled an Alabasta on us during Thriller Bark. And had a few light hearted episodes where we get to know Brook and Franky and how they interact with the crew. Similar to how they did this with Robin in Alabasta.
Honestly, Thriller Bark had some serious pacing issues in the opening episodes (too much of Nami, Usopp, and Chopper in Scooby-Doo mode) but after about 5 or so episode it seems to have found its grounding and pulled off some excellent moments once the melee between the Strawhats and Oars started. That's why I always cite Thriller Bark as the pinnacle of the new animation style because I felt it was the best consistently animated arc (and sadly the last one to use Eisaku Inoue's talents).
Then I was hoping for a mid size filler arc, and a large one. I don't really care what they did because, I just wanted some padding. I personally would've loved to see Unlimited Adventure as a fillar arc. We would get to see some fan favorites again like Smoker, or Crocodile; and it wasn't a bad plot. It fit the show.
I definitely think that Unlimited Adventure would have been a great season long filler arc but I actually like that One Piece is trying to avoid what happened with Naruto and Bleach and enter into its own personal filler hell because it does derail the story if it goes on long enough.
The Thriller Bark-Sabaody gap should've been much, much larger. Three episodes was NOT enough, considering the path from Sabaody to the current manga chapter leaves very little room for filler.
I think they should've gave us a 20-30 episode gap there. That would've put the manga far enough ahead that Impel Down and Marineford could have 2-chapter episodes, or even chapter-and-a-half episodes.
The advantages would've been great too. More Brook time. More Franky time. Could have given us some neat villains, etc.
While I felt that Season Two Voyage Seven was a nice breather I personally do not think One Piece should be too filler heavy nor should it give us more original interactions with underdeveloped crew members. Robin for her part got 13+ episodes, Movies 4-7, and two TV specials before we really hit her backstory and one of those TV specials resulted in a really OOC moment in an attempt to make her as humorous as the other Straw Hats. Granted Franky and Brook are far more extroverted and thus easier to handle but I think the filler we DID get was good enough. Plus, you already have fans complaining about Zoro accidentally cutting steal and certain details of the Straw Hats filler during Impel Down I think more filler would just add to that.
But if the issue is poor pacing I don't blame the distance between and anime and manga. I blame the director not being able to create the desired effect without turning whole episodes into staring contests. But hey, these are the same people who animated DBZ so…yeah...
O-chan