I haven't finished this game and probably won't for a while yet, and though I'm happy with it overall and have lately gotten really hooked, I'm wondering if a different backstory might have made some of the interactions and choices of quests feel a little more natural.
I can actually go either way on the whole "shouldn't the Survivor be focused solely on a desperate search for Shaun?" question. While it's plausible to react that way, it's also the case that Boston was the Survivor's home and a lot of people besides Shaun are looking for help at the moment, and that (s)he's walked into a chaotic situation where Shaun could be almost anywhere and charging headfirst into everything could easily just get a lot of people killed and leave Shaun no better off. Still, I'm probably metagaming a little to justify doing side quests and getting involved with the various factions because I, the player, know that this is an open-world Fallout game and I'll be missing a lot if I just focus on the main quest. (And while the intro was adequate, it didn't give me a particularly strong sense of this family and their relationships.)
The romance option feels a little out of place given that the death of the Survivor's spouse is still pretty recent in his/her mind. Out of curiosity, I decided to pursue it with Piper after *not* choosing the "Flirt" options the first two times they appeared, thinking that the game might lock me out of it if I didn't do something soon. She's funny and smart, and she's a good match for my character in that I'm playing him as wanting to help the Commonwealth population in general beyond just looking for Shaun. But I'd expect him to be still carrying too much grief for his wife to be interested in a new relationship, and in turn I'd think that Piper, who knows his backstory, would be a little more hesitant over whether this is healthy for either of them. Instead it's almost as if they've both forgotten that he's only been awake a short time since seeing his wife murdered.
The Lone Wanderer and the Courier, whatever the flaws in how they were written, at least have a purpose that allows the player to choose how quickly and single-mindedly to pursue the main quest. The Lone Wanderer is looking for a family member, yes, but that family member is an advlt whom you may soon realize probably lied about his origins and has survived in the Wasteland before, and meanwhile plenty of other people are asking for assistance with more immediate dangers. Plus, the LW *had* to flee Vault 101, so whether (s)he thinks Dad is in trouble or not, (s)he still has to find some way to survive and make a living in the Wasteland.
With the Courier, I never quite bought into the "I have to find Benny" thing in the first place. Running after someone who already tried to kill me once and who can only be found in a building where I have to hand over my weapons at the door seemed like a pretty dubious plan, and I didn't *need* Mr. House's platinum chip contract to survive given that I seemed to be finding plenty of other jobs and loot along the way. If anything, there was too *little* motivation for the main quest, as I didn't feel like I was delaying anything very important by poking around and helping the various settlements rather than going to the Strip, and some of the political and/or moral choices that arise later didn't sit right with me either. Still, I felt like I had plenty of freedom to go at my own pace without coming up with too many rationalizations as to why I was doing X before Y.
Since it seems like all three games (I haven't played FO1 or FO2) start out with a more personal quest that expands into a story about shaping post-apocalyptic society, I wonder if Bethesda might have done better to dispense with the personal quest altogether. Instead of having the PC be a soldier or soldier's wife starting a family, why not a soldier who doesn't have a family but assumed a community leadership role in Boston after leaving the service? Maybe your first encounter upon leaving the Vault is a ghoulified friend from before the war being attacked by Raiders who explains just how chaotic the Commonwealth has become. If you're playing a character with a strong sense of civic responsibility, or at least whose marketable skill set consists mostly of sociopolitical savvy and combat readiness, that would probably be all the motivation needed to go around getting involved in everything. At the same time, you wouldn't be brought into the situation having just lost a spouse and on a search for a specific person who, for all you know, is still a helpless child.