Your question about what Discovery added that was different is a good one. But I got to say that what exactly did Voyager or Enterprise add that was all that different, besides better production values? Most of the Trek series have dealt with some of the same themes that Discovery did, keeping that show in the Trek tradition. What Discovery has I suppose is that it's really the first 21st century Trek series, with a look and feel that's supposed to appeal to contemporary audiences. Enterprise, IMO, was a holdover from the '80s-'90's Berman era, while Discovery is coming after the mostly successful Abrams-Lin films and critically praised genre shows like Battlestar Galactica, and some of the others I've previously mentioned. I can't say Discovery has offered much new in themes compared to other Treks, and I do think a debate about how well the show conveys the themes it does explore is a worthy one, though I wonder if Discovery is a show that is designed to some extent to hook new and younger fans who aren't that familiar with previous Treks and aren't inclined to sit through 700 something hours of the older series. The idea of returning to themes, remixing them, or putting a contemporary spin on them, isn't something unique to Trek.
I think it's absolutely fair to say that VOY and early ENT were failures as shows over a much longer run than DIS had to date, insofar as they didn't try and add to Trek. That is why I don't think DIS is the worst Trek, even though the writing flaws were glaring at times. The show was always well acted, expertly directed, and the visuals (aside from the weirdly grainy ship shots which looked straight out of a early aughts video game) looked amazing. I never felt outright bored by Discovery, as I did by Voyager. Exasperated with the choices made some times, but not bored.
Discovery's production values, how it approaches diversity, and the willingness to have a harder edge does make it stand out from the other Trek shows.
Aside from finally having an explicitly LGBT character, I don't see what Discovery did better than earlier shows. I mean, looping back to DS9 again, O'Brien was the only white male human main cast member on the show. And even on LGBT issues, the show got about a half dozen references past the "Berman homophobic filter" without getting caught.
I really wish I could see what Fuller's vision for Discovery was and how it was changed after he left. I'm not sure I would've been on board with said vision though, if he's the one that wanted the Klingon re-designs for example. Not really a fan of that, though when it comes to behavior, the Discovery Klingons aren't that different than in previous Treks. I found Enterprise's take on the Vulcans more jarring and vexing.
I think people are a little too trigger happy to blame all of the problems of Season 1 on Fuller. I admit I haven't seen any of his post-Trek work, but he was a cut above most of the Voyager writing staff when he was young, and seemed to get Trek. I really wonder (albeit with little evidence) if he's succumbed to some addictive behavior in recent years, since he was essentially fired from three shows within a period of less than two years.
I do think the flow for Discovery Season 1 could've been better. They had this big opening and then a time jump and a swerve with Burnham being a prisoner. But the arc of her redemption I thought worked for the most part.
I think her arc worked great up through the end of Act 1. She transitioned from being shattered by her own failure into being a confident member of the team, and from a pariah to accepted. However, there was basically no arc in Act 2 - it was Burnham being emotionally tortured as she was betrayed in turn by Ash and Lorca, then irrationally latching onto MU Georgiou - basically showing that she had not made substantial character growth over the season.
I did feel that the final two episodes were tacked on, felt rushed, and felt like someone had told the writers to get the show back in line with some fans' expectations of what Trek should be, and as I've written before, the speeches about Federation values felt too obvious.
I concur. The editing in the speech seemed really, really weird. I do wonder if they wanted to have either a darker ending or to stretch the Klingon War into the second season, and CBS told them to put the kibosh on it. They had no plan B, so they engaged in pure asspull.
I wonder now if they should've opened the show with Klingon raid on Burnham's planet and then jump to her and Georgiou to better explain her mindset. Or intercut scenes of childhood terror with present day Burnham as she mutinied to better show the audience what was driving her. I do give the writers some credit for putting their lead character in such a position and then attempting to bring them back. It's like if Paris had been the lead character on Voyager. Though Chakotay perhaps would be the better comparison. Paris sought at one time to save his own skin, whereas both Chakotay and Burnham flouted Starfleet rules for principled reasons-at least in their minds.
My own two cents are they should have toned down the epicness of the season. There's no reason to have Burnham blamed for the start of the Klingon War. Or to have her former captain be one of the most decorated in the Federation. Or to then put her on a ship which is instrumental in defeating the Klingons.
Basically take all of that out, and make Burnham an XO who made a really bad decision, got her captain killed, and have her dishonorably discharged (not imprisoned). But as the Klingon War heats up, Starfleet needs more warm bodies, and she's recruited back into the force. GIven CBS didn't want to spring for the big budget needed for a large cast to pull off an epic show, this tighter focus - the POV of a single crew on a not-so-amazing ship - would have allowed Burnham's personal story of redemption to come to the fore. It would also stop canon nerds from complaining about why we never heard of this awesomsauce woman before - because she was just another Starfleet officer doing the best she could.
Ultimately, the stakes that make us enjoy stories are personal ones. Get the characters right, and people will follow them anywhere. Get the characters wrong, and even spectacle of universe-destroying scope won't be enough.