@Jabberwok:
There's some danger of Sabo becoming simply Ace 2.0, especially because, as you pointed out, they've filled very similar roles in their introductory arc. However, I have to disagree with you regarding their personalities, because Sabo feels significantly darker.
Part of what made Ace instantly popular was how he so clearly resembled a more mature Luffy. Though he frequently showed responsibility, he also acted very carefree and laid back most of the time. The two brothers even shared quirks, with Luffy reflecting narcolepsy really only around his family.
Sabo, on the other hand, is much more business oriented and shares fewer of those quirks with Luffy. He might flash moments of humor but in general he feels like a darker character. Seriously, just look at his face any time he's attacking someone. There's something a bit messed up inside his head. While both Sabo and Luffy obviously see each other as brothers, they lack the extended time together that made Luffy and Ace so similar. Instead, Sabo spent his youth in the care of the apparently more grim Dragon and exposed to the life-and-death struggle of the Revolutionaries on a daily basis, and that still weighs on him.
Sabo only feels so similar to Ace because Oda's been hitting us over the head with the comparisons, kind of like how Zoro pretended he was Mihawk for the first 60 chapters after the timeskip. It's a valid complaint if he's only a darker and edgier version of Ace to match a more serious second half of the story, but I think there are enough differences that Sabo's story will hopefully go in a different, more meaningful direction.
Oh, I never said that Sabos personality was a copypaste of Ace, I agree that they're very different. I just think Sabo has been used in an extremely similar way to Ace: The veteran coolio big bro who gets to help out his little bro and show what being really veteran and really really cool looks like.
Or to put in another way, its kinda like something you'd see in a long running sitcom; like, lets say Homer befriends some new character in season 21, who introduces him to a new interest, which he then obsesses over, putting a strain on his family. Its a new, character, a new interest, some new jokes….but you just can't help but think "I liked this story better when they did it in season 6".
--- Update From New Post Merge ---
@RamistaR:
At least we know that the similarities will not last forever. Ace is dead so there is a limit to the things Oda can copy-paste. A party on Baggy's ship, a fight with Blackbeard and an execution at the marine HQ. If Sabo survives this last parallel, Oda will have nothing else to copy-paste.
I could easily see the final World War featuring a scene where Akainu moves in for the kill on Sabo, only Luffy blocks it at the last second, because Symbolism!!
–- Update From New Post Merge ---
@King:
But then what about Ace's tattoo?
Ace was drunk and/or dumb and got his name misspelled, because he's a goofy klutz. I've met guys in real life that had this happen to them.
This is going to come down to semantics on what exactly you consider "foreshadowing" to be, but I'm going to have to agree with aokijismissingleg here in that I don't think Aces tattoo qualifies. In the 400 chapters between the reveal of the tattoo and the first actual hints at Sabos existence with the third sake cup, no one took Aces tattoo to mean he was honoring a dead foster brother. Its useless as foreshadowing, because it did not in the slightest way suggest Sabo existed to the readers. Now, its impossible to say exactly when Oda cooked up Sabo beyond-at the very least- when the third sake cup was glimped, 30 chapters before the flashback, but the reveal of Aces tattoo is absolutely no guarantee that Sabo was "born" at that time, because at that time no significance was attached to the tattoo.
Its easy to, in hindsight believe that Sabo was planned all along because the Tat matches his flag, but thats much more easily explained as Oda retroactively incorporating an old detail to tie into a new storytelling element. Which is not "wrong" or means theres a plothole or anything, and Oda is mostly quite good at doing little callbacks like that to make the world feel connected, but it is not the same as foreshadowing, it doesn't mean that Eiichiro "When Vivi first appeared she wasn't a princess, the crew was just supposed to beat her and move on" Oda had planned out Sabo a decade in advance.
Same with Corazon, the marine: Its neat that his face is on that Marine color spread, right? But that doesn't mean the spread "foreshadowed" Corazon being a double agent. Oda may or may not have Corazons details finalized at that point, but theres absolutely no hints whatsoever to the readers that Random Marine Mugshot #43 is Doflamingos double-agent brother (especially not when theres another random marine with Doffys exact sunglasses). Oda could just as well have been working on the design of secret marine mole Corazon, and then picked an unused marine face from the spread because hey, thats neat attention to continuity.