TMP doesn't really leave anything unexplained when it comes to basics like that. Everything is explained fresh. TWOK likewise doesn't need 'Space Seed' in order to work, everything we need to learn about Kirk's and Khan's history is reestablished fresh. GENERATIONS makes no such concessions.
This thread prompted me to pull out Generations and give it a rewatch and try and watch it with an "I don't know anything" mindset. And I was struck by how much time the film wastes, not just on the prologue but on Data, as well.
There's a screenwriting truth that I heard years ago, and I've tried to keep it in mind with my own (non-screen)writing -- a scene needs to either advance plot or advance character, and ideally a scene should do both. Data's scenes advance his character, but there's only one that advances the film's main plot (when Soran attacks Geordi and Data). The rest advance Data's personal subplot (installing the emotion chip and his reaction to it), but that subplot is so disconnected from the main plot (figuring out what Soran is doing and stopping him) that it's a waste of the audience's time. I realize that it's there to entice Spiner to sign, but I feel like they could have written a subplot for Data that was more relevant to the main plot.
As for things the film doesn't explain, three stood out for me. First, it's never really clear that Geordi's blind. One could sort of infer that, from the torture scene, but, from a 2018 standpoint, ti would also be reasonable to assume that Geordi is into cybernetic modification and enhancement. Second, Deanna's role on the ship isn't clear, nor does the film explain that she's an empath and picked up on Picard's feelings early in the film; she just seems intuitively sensitive to Picard's moods, like she's his emotional support person. Third, it's not clear that Worf is the security chief; weapons officer, yes, in the battle with Lursa and B'Etor, but otherwise, he's Riker's muscle on away teams.
Lursa and B'Etor were mentioned as being poorly introduced above, and while I think their introduction as secondary antagonists is fine, I felt that their motivation and why they were secondary antagonists was very poorly handled. The film assumes that the viewer knows the Klingon mythology of TNG (mythology is terms of what the series did with Klingons, not in terms of the gods the Klingons worship) and where the duo fit into that and their history with the Enterprise-D. A couple of lines of dialogue -- "Our rightful place in the Empire was stolen from us" -- would have solved a lot of problems. Their real problem, though, is that the film treats them as Generic Bad Guy Klingons, in that role that any other Bad Guy Klingons could fill because they're familiar faces to the fans in the audience, and then takes them off the board forever. In retrospect, I can easily envision a role for Lursa and B'Etor in the final four seasons of Deep Space Nine when the Klingons become a major factor there.
I think of myself as one of the film's defenders -- I wrote an article for Titan's Star Trek Magazine almost ten years ago defending the film and arguing its merits -- but watching the film through fresh eyes, trying to see it not as a fan but as a newbie, I can see that it's a film for fans only, and it's difficult to imagine how the other stories considered (Moore & Braga's A versus D, Hurley's holodeck weirdness) would have been better in that regard.