Actually it premiered on July 1999, two years after OP. One Piece was already huge by then..
I like how he went on with the "highly informed" but inaccurate rant.
Yeah, Rave and Fairy Tail are CLEARLY borrowing from One Piece in style, character design/personality, and even paneling, but that doesn't mean it's bad or without their own merits, dude. Oda-sensei's style was HEAVILY borrowed from Dragonball and Lupin III-esque stuff prior to his assistant-phase.
Ironically, it seems like either he and Hiroyuki Takei influenced each other's style or he adapted a lot of Takei's style touches. Takei was influenced by graffiti and Tezuka, and it's always been my guess that Oda got the cartoony aesthetic idea for One Piece thanks to Takei(he has mentioned Takei as his friendly "rival" in manga creation).
Ohbata, the guy who does Soul Eater, is borrowing a lot from the OP-style, too. Thanks to OP, it's a popular trend to use simpler, more stylish cartoony-looking art as opposed to the grittiness, realism, and muscleheads from late 80's/early 90s shonen manga. Noone feels like Mashima is a THIEF or anything for borrowing from Oda. Well…they don't except for those times when he's reaaaally walking the line. FT wouldn't do as well as it does if it weren't interesting in its own right.
Togashi seems to borrow HEAVILY from whatever the most popular series are and just carves out his own niche by making everybody BISHIES. I mean, Yuyu Hakusho is EXTREMELY similar to Dragonball and Slam Dunk to the point that attacks, characters, and plotlines seem copy and pasted, but as long as you're adding some personal flair to it, it's not a big deal. When something is TOO derivative of another series it gets rejected by editors anyway(Happened to the original pitch for Bleach).
FTR, assisstants jobs depend on the specific manga-ka, but like everyone already mentioned, they usually just help out with inking speed-lines and backgrounds. They pretty much never touch the characters and sometimes aren't allowed to do anything besides help ink backgrounds and tone or letter. They usually never influence story aside from maybe a sounding board from what I gathered in Jump interviews.
Also authors tend to draw backgrounds and stuff, too most of the time or atleast SOME. It seems like the creator only lets assistants jump into draw stuff when they're behind deadline or it's especially overwhelming to finish drawing and inking everything by the deadline.