I loved New Vegas and kind of enjoyed 3. Thus far, Fallout 4 has has a few problems but overall I'm enjoying it quite a lot. Exploring is fun, the gunplay itself is much better than the previous games, building settlements is fun, and so on.
The UI is god-awful though, and I feel like I'm fighting against it whenever I take any action. I'm growing more accustomed to it as I go on, but it's far from intuitive.
Graphically it's mostly gorgeous, even if Bethesda is even worse at animations than Bioware. The lip-syncing is frequently off, facial animations don't do a good job of conveying emotion. That said, some things too look quite nice. Deathclaws zig-zagging to avoid your shots looks great, and the dog is pretty neat too. Player body animations are better than before but jumping looks weird. Elsewhere, there are some bugs with the skybox textures.
Pretty much I'm just having fun exploring, just like I did in every other Bethesda game ever. In that respect, Fallout 4 satisfies quite nicely. I haven't gotten too far into the main story yet, but since when has anyone ever played a sandbox RPG for the main story? Maybe that's not for you, but, for me, wandering on foot from place to place, finding new places, and occasionally getting some really cool new loot is very enjoyable. For your third point, I think you really are just thinking about it too much. It's fun to explore an interesting world, and the action really isn't as sparse as you make it out to be. Even in the short time I've played it so far I've been in quite a few very fun firefights, and I still remember some of my most memorable battles in New Vegas. Low level, poor equipment, making my way through a vault infested with raiders, pulling every trick I could to whittle down their health so I could get to the end. And then learning that, with the right armor and some speech skill, I could have just made my way straight to the end without fighting.
That's another thing I like about these RPGs is finding non-violent solutions. For some reason, being able to negotiate my way into a better outcome is very satisfying, and I love it when games do that. In stuff like Skyrim that's not as present, but in many sandbox RPGs it is, which I imagine is part of the reason for their popularity.