Boba Fett as upstart Tatooine crime lord. It's a good starting place for a plot, and throw in Fennec Shand as a right-hand woman and the already well-established legacy of the Hutts within the wider Star Wars canon and this show definitely had potential. Unfortunately, they decided to also include a bunch of back story to fill in Fett's timeline since Return of the Jedi, and then didn't give themselves enough time to actually flesh out the underworld that Fett has entered into.
Don't get me wrong, I quite enjoyed Fett's backstory. In fact, I think the way the Disney+ shows keep expanding on and diving into the culture of the Sand People is one of the cleverer and more interesting parts to them. But it's almost a more interesting story than what's happening in the "present". It doesn't help that the two overlap each other so much thematically: both arcs see Fett ultimately at war with the Pyke crime syndicate (galactic drug runners previously established in the Clone Wars series) and trying to protect his found family. Except he earns his place in the Tusken tribe through grit and determination, but back at Jabba's Palace we just get small, episodic stories about him being a "nice" crime lord and slowly building a group of loners and outsiders. It's just a bit formulaic and it never paints Boba as anything other than out of his depth. He keeps telling Fennec that he's "sick of taking dumb orders from idiots" and how he can clearly run a criminal enterprise much better than those he used to work for, but then the show never provides us with any evidence of this. Honestly, it feels like he's completely green, and without Fennec's constant counsel, would likely have lasted less than a week.
At the same time, whilst we develop a deep understanding of the Tuskens, we're never given much of a look into the rest of the Tatooine sphere of criminals. There's a nice moment where the heads of the various families (or "Daimyos") are brought in to broker a peace, but rather than giving us any insight into how this black economy operates, they're just showboated for a bit and then ultimately betray Fett. That's it, that's all the background or development the wider world gets. There's a cantina rip-off with a genuinely interesting owner, but then both the building and owner get blown up. There's a corrupt local Mayor (and spineless lackey) who turns out just to be an idiotic puppet, despite giving good advice earlier in the show. None of the pieces are ever more than surface-level, providing no real depth or intrigue to the wider plot.
Then, halfway through the series the show just becomes The Mandalorian, season three. Din Djarin comes back in, not just as a hired thug or even as a friend. No, we literally go to a different part of the galaxy and spend over an episode of total time filling in what he's been up, advancing his (unrelated) plotline, and even seeing him go meet Grogu, Luke Skywalker, R2-D2, and even Ahsoka. None of this is relevant to our central plot, but it takes up almost two full episodes – and there are only seven to begin with!
I'm not complaining, really, but it immediately shows up the central issue with The Book of Boba Fett. The moment Din walks on screen, the show gets more interesting. I was more committed to his sidequest to deliver Grogu some Star Wars brand mithril than the main arc back on Tatooine. His character development is much more significant and nuanced than anything Fett or Fennec receive, and his plot actively advances the wider universe and lore much more. Sure, back at Mos Espa Cad Bane pops up, and I like that the show closes off the rivalry between Bane and Fett (even if the former looks a little weird as a photorealistic character), but in the meantime Din is providing the audience with insight into Luke's new school, the fact that Ahsoka and Luke are friends, further backstory on the Dark Sabre, and Din's expulsion from the order of Mandalorians that he was raised within. It's just way more impactful and interesting.
Which isn't to say that the main thread is boring or even plain bad, but just that it never leaves the shadow of the show it has spun off from. It absolutely could have, but it's never given enough space to breathe and explore its own story. Instead, it serves as a way to advance the Mandalorian plot, tie-off various loose ends from the Clone Wars series, and fill in Fett's missing timeline. Honestly, if Temuera Morrison and Ming-Na Wen weren't excellent in their respective roles, the Fett arc would be extremely bland. Sure, we get Fett riding a Rancor (which is pretty cool to see and isn't as gratuitous as needed), but otherwise there's little payoff and it all just feels a bit flat and pointless. It's the TV equivalent of vanilla ice cream: it's not bad, and at moments can both stand on its own and massively improve other parts of a dessert. But the moment you pair it with anything even slightly more interesting, it just becomes a side, filler food that supports the main act but never takes the spotlight.