Just finished reading ch 584...
First of all, I need to address the HUGE character death that just happened previously. ACEEE!!!!! NOOOOOO!!!!
Well, having a major character death in the story makes it interesting and the twist was something no one really expected because usually authors do not usually kill off their characters that they have spent so much time and effort creating (the only other major character death that I can think of at the top of my head is L from Deathnote...). The fact that Oda had killed off Ace allows the series to gain more depth and acts as a hook that pulls the audience's fascination.
An interesting thing to note is that I don't think there has been any real character deaths in the entire series, other than Gol D. Roger's, but his was a necessary one so that the One Piece series could be realized. Without his death, the story would probably never have taken place. Therefore, overall, his death is the point that everything begins.
Anyway, on towards ch584. I really don't have anything to talk about in this recent chapter but for the fact that it is nice to finally see Ace's and Luffy's childhood together and how they met. I was quite shocked when I found out that they are not blood-brothers and that Ace is the son of Roger's.
In this particular chapter, the reader finds out that Luffy, although already posessing his determination and willpower that distinguishes him from the many of the other current pirates, does not yet have the strength that he is known to have when he is older. He cries about the pain he is in when he is being beaten up with spiked gloves, which is quite different from how he is when Older!Luffy is fighting. I personally feel that it is nice there is evident character development all the while the willpower he presents to the world as Older!Luffy is already present within him as Younger!Luffy.
A last point I should make is that I'm still wondering who Sabo is and where he went, since it seems to indicate that the three of them became best of friends in their childhood.