Okay, before anyone jumps down my throat, just hear me out. I have discovered what I believe to be F4's largest flaw.
Due to over-reliance on the Radiant Quest system Bethesda is so proud of, F4 is boring.
It isn't the poor writing, it isn't the half-baked mechanics, it is the nearly fetishistic usage of the radiant system for everything, combined with a distinctive lack of variability in that system that is the game's ultimate failure.
The side quests are actually halfway decent (mechanically, the writing is still crap), the main quest, in spite of being an absolute hack job, is playable. But the incredibly limited variety and vast over-presence of Radiant quests just drags the whole game down abysmally.
I've put in 200 hours on my main save, and while I have yet to even bother completing many sidequests, the Radiant system has utterly burnt me out when it comes to basic gameplay. (I may just shoot Garvey the next time he tells me that the settlement I just resolved the problems of has more problems.)
Honestly, I was beginning to think the same thing. The main quest is pretty short, and if you weren't being distracted by Preston giving you quests to go kill 10 ghouls or raiders every time you finished one, the game wouldn't last very long at all. It took me quite a while to realize that the Minutemen quests never ended.
At the same time, while these quests and other side quests do give some variety, I was getting kind of annoyed by being distracted by them. I continuously kept close to 10 quests in my Pipboy at a time, and it seemed like I couldn't finish something without another quest or two popping up. Or, more commonly, finding another landmark infested with super mutants or ghouls that I had to explore to find the not very useful legendary weapon inside of. I was kind of hoping for better pacing there, where you discovered a few more focused quests at a time as you followed the main quest.
Speaking of legendaries, the whole "Legendary Enemy" thing is kind of immersion breaking. I'm not saying that they turned Fallout 4 into an MMO, but it feels like they thought about it. Oh, look, random special enemies that are guaranteed rare drops, and who "mutate" halfway through the fight, whatever that means! The first time I saw that was with a super mutant, then a ghoul. Okay, those can mutate. But raiders? That felt a little strange.
I guess I have a lot of small gripes with the game, but the fact that it's monotonous and kind of boring is definitely a big thing I'd started to notice.