Man if you skip chapter 1 or the romance dawn arc you miss the significance of Shanks, how Luffy gets his powers, devil fruits, how Luffy gets the hat. Luffy meeting Coby, how he recruits Zoro and Zoro's backstory.
Skip Orange town you miss Buggy, more info on Devil Fruits, how Luffy meets Nami, the foreshadowing of Rayleigh and what crew Buggy and Shanks were on.
Skip Syrup Village you miss Captain Kuro, the fact the marine he had hypnotized was Captain Morgan. You miss Ussop's backstory and the fact his dad is on Shank's crew, and you miss how they get the Going Merry.
Skip Baratie you miss Sanji's recruitment and backstory, the fact that the 1st half of the grand line is called Paradise, Mihawk, the event that changes Zoro's personality, and Luffy vs Krieg.
Then you have Arlong park where you learn about fishmen, foreshadowing to Jinbe, Nami's backstory, more marine corruption
Loguetown you get Luffy's near execution, introduction to Smoker and logia fruits, Dragon.
Then reverse mountain has Laboon who sets up for Brook down the road and Crocus
Whiskey Peak introduces Robin and sets the stage for Baroque works and we are told of the Warlord system.
Little Garden you learn about Elbaf and meet Dorry and Broggy who are important when Enies Lobby comes around, plus it's the point where the crew has to find a doctor.
Drum Island you get Chopper, zoan fruits, Dr. Kureha, Wapol who is important for making wapol metal, and you learn about the Revire, plus you get the great speech by Hiraluk during choppers backstory. and a small hint about the importance of the D initial and Blackbeard is first mentioned.
Alabasta you get Crocodile, information that devil fruits can continually be trained and enhanced, find out about poneglyphs, and the ancient weapons and the strawhats now have their first major royal ally. Also Ace gets introduced and hints of Whitebeard and the fact Ace is chasing blackbeard, Sanji's Mr. Prince persona, and devil fruits can be given to objects, and the fact that their is a Void Century in history
Jaya sets up skypeia, plus blackbeard and bellamy's intro. Plus the story of Montbloc Norland, introduction of whitebeard, and the summit of warlords shows Sengoku, Mihawk, Kuma, and Doffy.
Skypeia tells about Sky Islands, Observation Haki aka Mantra, the dials allow Ussop to get a power boost and it lets him upgrade the climatact, Zoro learns flying slashes, foreshadowing of the Klabautermann, Enel, more info on Norland, more info on Roger and the poneglyphs, natural weaknesses for devil fruits besides water and sea stone.
Long Ring Long Land- Afro Luffy and it at the end of the arc Luffy meets Aokiji and realizes he needs to step things up which leads to him coming up with Gear 2, more info on Robin, and Aokiji freezes the ocean to let the old guy get back to his tribe and so he can ride his back on the ocean, this leads to the extremely violent Aqua Laguna due to Aokiji's ice melting and adding extra water.
Water 7- Robin leaving, Ussop vs Luffy, Franky's intro and backstory, CP9, Merry being unable to sale, Ussop leaving, sea train, Tom, more government corruption, Luffy learns Soru, the beginning of the tonal shift of the series
Enies Lobby- Robin's backstory, Gear 2 and 3, Asura, Diable Jamble, the fact Luffy is pushing himself further and further to defeat enemies, Buster call, and Merry's Funeral.
Post Enies Lobby- Garp, Bounty raises, Ace vs Blackbeard, Thousand Sunny, info on the Yonkou and intro on the New World and Vegapunk gets mentioned, Luffy's father is mentioned
Thriller Bark- Brook, tie in to Laboon, Moriah, Kuma and Zoro, the fact Moriah lost to Kaidou, Ryuma and how Zoro gets Shusui, vivre cards, Luffy gets captain john marker
Saboady- 11 supernovas, Camie, the return of Hachi from Arlong park, Duval, Celestial dragons are introduced, slave trading and hints of the underworld run by Doffy, Pacifistas, Strawhat separation, meeting Rayleigh, Kizaru, 1st hint of armourment haki, first hint of Luffy getting conquers Haki.
Amazon Lily- Boa Hancock, alerting Luffy of Ace's execution, island of haki users
Impel Down- Buggy returns, Magellan is introduced, more world building, Jinbe is introduced, Ivankov, Mr. 2 returns, Crocodile returns, etc.
Marineford- 1st time we see top tiers really fighting, Whitebeard, Akainu, lots of character introductions, reveal of Ace's father, etc.
Post Marineford- Law saves Luffy, Luffy's backstory, Rayleigh comes help Luffy to plan for 3d2y