Filler arcs are common in shonen anime. When the anime runs out of source material to adapt, the studio will create its own story to fill time until the manga reaches a point that the adaptation can continue. One Piece is a rare exception where there isn't that much filler. However, many episodes have content padded, mixing in filler with canon material. This slows the pace considerably, and, for a lot of fans, makes the anime in particular nearly unwatchable at times.
One of the earlier arcs in the story, Skypiea, has garnered something of a reputation among fans as being skippable for one reason or another, treating it like filler. Unlike actual filler, however, the Skypiea arc is entirely canon -- so what about it makes some fans want to skip past it?
The Problems With the Skypiea Arc in One Piece
The adventure on Skypiea feels like something of a side quest for the Straw Hat crew, who go entirely out of their way to reach the island above the clouds. Visiting the island does nothing to actually advance their journey through the Grand Line, and it can feel like nothing more than a diversion as a result.
The Skypieans, even Enel, the main antagonist of the arc, haven't ever really been shown again, despite their significant roles in the arc. The story moves on without their presence just fine. However, although this arc might feel like canon filler, it could be one of the most important arcs in the overall series. In the prelude to Skypiea, the Straw Hats go to the port town of Jaya, where they first learn of the sky island. It's here that Luffy first encounters Blackbeard, who later becomes one of the main antagonists in the series overall and is thought to be Luffy's final opponent in the story. Luffy also squares off against Bellamy, a subordinate of Doflamingo.
One Piece's Skypiea Arc Sets Up a Lot of Significant Developments
Without Blackbeard's introduction, the rivalry between him and Luffy wouldn't make sense, and without Bellamy, the Dressrosa arc would lose a significant plot line. Both are integral to future arcs, and the Skypiea arc itself hasn't even begun yet. On Skypiea, the Straw Hats learn many powerful abilities that later become staples in their arsenal. Zoro learns how to fire his sword slashes through the air, Usopp discovers the technology of dials and uses them to upgrade Nami's Climatact staff, and most importantly, Haki is shown for the first time, albeit under a different name. Enel and his guards use what they called Mantra, the Skypiean name for Haki. They only knew how to use what is now known as Observation Haki, but this is an important step for the introduction of one of the core power systems in One Piece.
A number of mysteries also begin in this arc. Robin, when separated from the crew, discovers a poneglyph in the middle of an ancient city on the sky island. The poneglyph reveals the location of one of the three ancient weapons, Poseidon, and also has an inscription from the King of the Pirates himself, Gol D. Roger.
The Skypiea Arc Epitomizes One Piece Fun
This tells the Straw Hats that they are following in Roger's footsteps, which begs the question: why did Roger come to Skypiea in the first place? The answer turns out to be quite simple: because they wanted to. Just like the Straw Hats, the Roger Pirates took a detour to go to the sky island because they wanted to go on an adventure above the clouds. They wanted to have fun. This fact in and of itself is one of the prime examples of why this arc shouldn't be skipped by fans of the series.
The Straw Hats and the Roger Pirates just wanted to have some fun, and that's exactly what the Skypiea arc is. Skypiea is home to some of the funniest moments in the series, with lots of great gags, face faults and so much more. Even with all the mysteries and reveals stripped out, the arc itself is pure and simple fun -- just as a story about a pirate adventure should be. The entirety of One Piece is about a grand adventure to the end of the world and experiencing everything within it along the way. Fans who skip out on an arc integral to the story miss out on the full experience, and are therefore missing out on one of the most enjoyable arcs in the story.