Warning: Minor Spoilers
If I were to say anything bad (and I will have to do that as this review is not a positive one) about this anime that would not offend the hardcore fans (and I know there are a lot) is that it's PAINFULLY slow.
Even if you leave the fillers aside, still- the plot itself progresses in such a slow pace that it's rather hard to watch. Now for a younger audience it may not matter that much, but to me as a higher age audience, it matters greatly, after all, out of the 23~ minutes of each episode 3 are spent on opening and ending (which is usual I guess), but in most episodes, roughly 5 (!) or more minutes are spent on a recap of the current story arc, as if the creators of the show forgot it's a shounen anime and instead got the idea that the audience are elderly people with Alzheimer.
But ok, this means there is still 15 minutes left per episode right? Well, not really. In most episodes, the heroes get separated. When they do, the anime tries to keep track of ALL of them simultaneously, cutting every few minutes. This wouldn't be so bad IF after the cut, the same exact scene did not repeat itself (example - a few seconds that show one of the main heroes standing and looking at a place/ villain that is shown from a few angles. Then the anime continues elsewhere, and when it comes back to the same place, the same exact scene is shown). That may not sound as much, but in drastic cases that can sum up to a few minutes (the worst case I bothered to count was almost 4 minutes of time wasted on watching the same scene). Combined with a lot of other tricks, I'd say that a lot of episodes are left with about 11 minutes of actual content on average. That is simply dreadful.
Now to say some positive things for contrast- the story is actually very interesting and the characters are well done... BUT, no matter how good something is, if you drag it out too much (and this is indeed the case with One Piece), even the best story in the world and the best characters possible (not to say one piece has neither of those.. The story is great, and the characters are good, but not more) will not keep you entertained, and turn even the best story possible into a boring experience.
Now back to the negatives. NO ONE DIES IN THIS ANIME. This is of course an exaggeration, as some people actually do die in this anime BUT only when it's critical to the plot. At all the other times, the characters "die" just to add drama and to come back a few scenes later.. Sometimes to "die" again and add even more "drama" (added "" because at this point its more awkward/silly after one time rather than dramatic). The worst case of this I saw was with Brownbeard who "died" something like 5 times over 2-3 episodes (if I remember right), and still stayed alive in the end while the show tried to suck as much drama out of this as possible. And this brings me to the next point- fake drama. They seem to try and squeeze out as much of it as possible, at every opportunity they get. From crying children (and adults) to characters repeatedly "dying" and to the most cliche tearjerkers possible at every turn of the story. They even go as far as to try and make the villain's henchmen AND EVEN THE VILLAIN himself have a tragic past that you are supposed to relate to and feel sorry for (because seems a villain can't just be evil because that is just the way he is, he had to become evil because something bad happened (lol, how about sheer ignorance or plain greed for wealth or power?)... and we apparently MUST know. for both villains and side characters that we'll never even see again). Also I mentioned crying.. There is an unbelievable amount of it in this anime. And that would be ok (I guess) if
A- they did not try to milk it for drama (yep, that's a word that sure gets a lot of use when discussing this anime huh?) every single time
B- they didn't think that when someone cries, he instantly turns ugly, with snot running down his nose and with the worst face expression possible, regardless of gender, age or how emotional he is.
I'd say a lot more, but there is already a lot of text, and it's enough to sum it up - this anime is not one I would recommend someone to watch under any normal circumstances (maybe only if someone made a shortened version with all the repeats, openings, endings and cuts removed thus making the whole anime be at around 300 episodes max), I watched over 600 episodes myself, and I can safely say that although some parts were enjoyable, it was mostly rather boring when looked as a whole, and especially the later episodes, and I am regretting the time I spent watching it (as to why i watched it so far- my friend recommended it to me and I did not expect it to last this long. I usually finish what I start, but this is ridiculous, bad, and really beyond me at this point.)