I've never really understood people's dislike for Ace, but I'm thinking the same towards Sabo is even weirder. He's a little more menacing than I might have expected - which appears to be the root of what bothers most people - but other than that, he's bubbly and dynamic, and his past isn't wasted. We got all of that in about half a chapter of a fight scene which was mainly Oda showing off Sabo's fighting style.
All I can really see in anti-Sabo posts is people who are really determined to hate Sabo doing their best to hate Sabo.
You pretty much hit the hammer on the head for me. I think all of this started with the revelation that the Mera Mera no Mi was Doffy's prize at the colosseum. Doffy says that Straw Hat wouldn't be able to refuse the bait that he placed, and he was right. It was his brother's fruit after all, it's as much a part of Ace as anything.
So the question here is how the story could have gone. The problem is that, anyone who eats the fruit that isn't on Luffy's side automatically becomes villainous in his, and probably our, eyes. I could be wrong, but even if someone like Cavendish were to win the fruit, there would be some resentment there since it was his brother's fruit. That character would definitely have to be setup as a major player later on, too, cause otherwise it'd be a waste of the Mera Mera. If any non SH was going to eat the fruit, it was definitely going to be Burgess. He had interest in it and so did BB. Luffy even said he was basically trying to fight for it to get it away from them. The problem is if Burgess eats it, Luffy now has two people to resent in the Blackbeard crew. Teach for capturing his brother, and Burgess for eating the fruit. Even if Luffy won, who on the Straw Hats would have eaten the fruit? Probably not anyone, as it would have changed their established fight style way too much. Plus, it wouldn't gel well cause no matter who eats it on the SH side they will end up feeling like a copy of Ace in some way, it would change their character. Blackbeard said Burgess eating the fruit is like getting Ace to join his crew, and he's right about that.
So I think everybody already knows this. Sabo comes out of left field, literally out of nowhere to solve this problem. Him being there kills two birds with one stone. It lets Luffy focus on what they were really on Dressrosa for, and lets someone eat the fruit that both Luffy, and more importantly the reader, can support. We don't mind if Sabo eats the fruit, and neither does Luffy. Of course there's going to be comparisons between Sabo and Ace, because at the end of the day anyone who ate that fruit would have been compared in some way. Sabo even says "I'll never hand the Mera Mera to you, Strawhat Luffy!"
For a reason we're not sure of, and may be explained at the end of the arc, Sabo wanted the fruit. He wouldn't even let Luffy have it. His desire for it could have even superseded their mission of trying to get dirt on Dressrosa's illicit activities. Sabo says he will be the one to "carry on Ace's will". Of course, inherited Will is a major theme in this story. He may have wanted Ace's fruit to compensate somehow by not being able to be there for him during Marineford. Even though Sabo showing up was really weird and came out of nowhere at first, it's more about the emotions of the characters and emotions of the reader. Luffy at least has one brother left, one he can entrust to pass Ace's fruit onto.
This all comes home for me during Sabo's talk with Fujitora. It's notable that in the Grey Kingdom backstory, Ace only cried as a kid once he found out that Sabo (presumably) died, after all that was his brother. Before this, he told Luffy that men don't cry. So the opposite happening with Sabo breaking down in the flashback, crying over Ace is a great moment for me. It's less about his physical fight with the Marines, and more about the emotions of the characters in my opinion. He wasn't there for Ace, but he'll make damn sure he's there for Luffy, no matter what, even if he has to confront a Marine admiral. To simply disregard Sabo as an Ace clone I feel is doing injustice to both of their characters. From the perspective of the story, Ace in the past was set up to be a rival to Luffy. He said he would make Whitebeard the Pirate King, not Luffy. In that regard he was somewhat similar to Shanks, acting as both someone Luffy could look up to, and one day surpass.
With Sabo there's no such relationship. He's not a pirate, he works for Luffy's dad. He's also another member besides Ivankov and Dragon that ties Luffy to the revolutionaries as a whole. We haven't actually seen too much of the revolutionaries yet, they've felt like a tertiary aspect to the story ever since the beginning, despite having some key members we've already seen. They're definitely being built up for something as the story goes on, so Sabo being alive and part of them just adds another layer to that. Ace and Sabo have similarities, but I feel at their core they're different characters. They're both confident, probably a little cocky, and protective of their little brother. Still, Ace always had a temper to him, and seemed somewhat stubborn. He also was easily provoked by insults and usually reacted with violence. So far, Sabo seems to be pretty even tempered, and I'd argue he's more similar character-wise to Luffy than he is to Ace.