3) Bad Animation and Art - Look at some of the shorter shounen adaptation by BONES, Madhouse and you'll see why One Piece horribly fails here. It comes with the series being a long running series, yes, but madhouse did show with HxH 2011 that you can have a long running show AND make it look amazing too.
I do want to say here, though, that because One Piece is by Toei and is also always going to be one of the longest Shounen in existence with a pretty consistent schedule, it comes with the territory- but not always.
Toei does a lot of outsourcing for its animation because of the costs involved and the other shows on its plate (such as, right now, Dragon Ball Super). With this in mind, it does actually have some pretty fantastic animators that work on it every now and again. Naoki Tate, for instance, who's been doing
a bang-up job in Super, does some
pretty fine work, especially where character acting is concerned. Similar praise can be given to Ryo Onishi for his
expressive animation and
impactful fight scenes.
I won't say this as fact, of course, but I do think that it really is the size of the series as well as the time that it would take to make well-animated segments consistently that does lead to its inconsistent art and the sub-par impression that it leaves. Comes with the territory considering the animation staff itself is rather inconsistent, after all. This in mind, the show can have some pretty damn great animation when the studio sees it as necessary and, I would say, if you really want to experience it visually take a gander at the movies (which I haven't seen but have seen the sakuga of).
...That said, the manga's art is pretty consistent, chock full of detail, and just plain good, so I'll always be an advocate for that, just wanted to expand on the bit about its visuals to give a bit of context as to why it really couldn't consistently meet the aesthetic candy that comes from a Bones or a Madhouse.