Whew, now that the bile is out of my system, let's talk about something far more interesting - One Piece!
While the One Piece anime is one of the better adaptations, it still has quite a few additions and subtractions. Some of them, I would submit, even serve to make the show better, most are fairly irrelevant one way or another, and a few actually make a negative difference.
The biggest difference lies in the cover page arcs that Oda does in the manga - he typically uses that page to tell an ongoing story of what happens to the characters that the crew leaves behind. Sometimes it's just funny, or nice to know where someone ended up, but increasingly, they've been an important source of information, sometimes even critical information. (They did animate two of those arcs, but the viewer numbers were apparently terrible - for some reason, people liked fillers about the crew better than finding out important information><)
There are some changes I'm just going to leave out as they not important at all (does it really matter that Luffy fight's Helmeppo in the street instead of in the restaurant?)
Episodes 1-19, Chapters 1-41
Most of the changes here are just in the order of telling.
The manga opens up with Luffy as a child dealing with Shank's crew, and then moves on to the encounter with Alvida.
Luffy's barrel get's washed up on the island where Alvida's pirates hang out, instead of a ship.
Nami doesn't show up
Zoro has his flashback at the Naval base, instead of much later
Nami stealing the pirates' boat happens off-screen
Buggy kills the crew member that insulted him with a Buggy Ball instead of letting him go
It's a little more clear that Nami intended to get Luffy away from Buggy after she had stolen the key a second time
Zoro's response to being kicked in his wound is a lot more graphic (go anime!)
They meet Gaimon before Usopp - also, Zoro is asleep the whole time, so Nami and Luffy are the only ones that actually talk to Gaimon
It takes longer for Luffy to realize that Usopp is Yasopp's son; he also tells him when they're alone, so Nami and Zoro don't know.
Cover arcs - the Little Buggy adventures, which are covered in the anime
Episodes 20-30 Chapters 42-68
There's an explanation of why Johnny and Yosaku thought Mihawk was at Baratie - there was a customer there called Red-Eyes (Akanami), not Hawk-Eyes (Takanami)
Johnny and Yosaku attack Fullbody when he insults them - getting beaten quickly
During the Sanji Flashback, Zeff eats his own leg to survive instead of losing it to save him from drowning (anime explanation is much better :p)
Cover arc - the Little Buggy adventure, covered in the anime
Episodes 31-44 Chapters 69-95
Here is where the anime starts to really faithfully follow the manga in detail. The differences really become "complete filler scenes" for the most part.
Nezumi orders his men to fire, instead of doing the shooting himself
Cover Arc - Morgan's Escape, covered in the anime
Episodes 45-61 chapters 95-100
A ton of filler here, as the episode counts vs chapter count shows :p
The encounter with Fullbody
Luffy's encounter in the bar
Zoro breaking Tashigi's glasses and the stuff that follows. In the manga, he just picks up her glasses and hands them back
The cooking contest
The fight with Smoker where Luffy accidentally flings himself away
The Daddy Masterson story is technically filler, but it was actually written by Oda and cut from the manga because they wanted chapter 100 to be the beginning of the Grand Line
It's strongly implied in the manga that the insane wind is caused by Dragon - someone yells out a command (I've seen it translated as "Storm" and "Wind Blast") right before the wind picks up.
The Going Merry doesn't drift away, so Luffy doesn't do the Gomu Gomu Rocket
Everything with Apsis and the dragons.
- Dragon has an additional line in the anime "Wind, blow. Bring forth the Storm of Destiny!" as he's walking the streets
Eric doesn't show up to spoil the ride up Reverse Mountain
Cover arc - Morgan's Escape
Episodes 62-90 Chapters 101-154
Mr 3 uses wax swords to pin Brogy down instead of shackles
Nami's bug bite is shown as a flashback on Drum island
Cover arc - Jango's Dance Paradise - Jango is left behind and then flees to a dance island, where a huge dance contest is going on. He and Fullbody compete for first place, Jango wins, but they strike up a manly friendship and then together they beat a group of rampaging pirates - Jango's cover is blown though, and he is arrested. Fullbody fights to free him though, and wins but earns himself a demotion (Objection Dance ftw) Just as Jango is about to erase Fullbody's memories and sail away, he catches sight of Hina - he and Fullbody both vow to follow her forever.
Episodes 90-143 Chapters 154-218
Another arc with a fair amount of filler mixed in
The scenes with Luffy discovering Dance Powder early
The scene where Chopper meets Eyelashes
Anything with Ace after he destroys the Billion's fleet - in the manga he leaves right after giving Luffy the mysterious piece of paper
98, 99, 100 and 101 are all filler
Episodes 131 until the very end of 143 are all filler (until the ship falls from the sky)
Also, most of Mr. 2's references to "Okama" replaced with "Ballet"
Cover Arc - Hachi's Walk on the Sea Floor - Hachi escapes the Navy and adventures under the ocean rescuing various sealife, including Keimi the mermaid and her friend Pappug the starfish. However, he ends up trading them to a group of his old crewmembers (pre-Arlong split) for a map to the tastiest takoyaki recipe that he wants so he can propose to his old love, Octopako, who rejected him for his terrible takoyaki. He follows the map, but believes he's been tricked, so he goes and beats up his old friends and frees Keimi. But., it turns out that they didn't trick him and he gets the recipie - anyway long story short, he ends up deciding to reject his old love in order to do the right thing and sets up a takoyaki stand with Keimi and Pappug.
More to come!