Never played a Dynasty Warriors game before this but I can comment on the story.
There is a main story ("Main Log") mode that recaps most of the story so far through narration and a few cut-scenes as you play through the levels. It leaves out a few arcs and tons of details (for the sake of time), but if you pay attention you should be able to get the gist of it. If you were planning on reading/watching more of One Piece then expect this game to spoil you on some big plot points.
i havent played dynasty warriors since 4 on the ps2. (i tried one later and they had rebooted the fight system to something completely unenjoyable)
The musou part of the game plays much like sengoku basara. its not really an open map as much as it is many rooms connected together and when u kill enough people in each room you take over that area and move it. its fun tho.
in main log the game has action modes where it is more like a third person adventure game. the story has only the highlights and recreations of epic scenes but leaves a lot of spaces and red herrings to anyone who has not scene the show. if you like the show I'd suggest only playing up to where you last watched and enjoy the anime/manga first. if you dont like the show then just skip the scenes cause it wont make any sense anyway.
The story mode is about as good as Gundam's and FotNS, but unlike those, it doesn't have any fictional scenario fun to help flesh out the game so a lot of side characters end up with one or two stages. The combat is a lot of fun though, but overall the game just isn't as good as other recent musou games. I'm sure a sequel will help.
Above Warriors of Troy, lower than just about everything else.
My biggest complaint is that you have to rely on the "coin" system, which could have been cool, but was implemented badly. However, I like to have my cake and eat it too, which is why I think that SW2 had the best skill system.
This is no Zaku, boy! No Zaku! XBL: Maurigan PSN: Fioyl