A diamond in the rough.
Aight, I'm going to start with the negatives.
Frankly, the writing has rather awkward sentence structuring, and while I don't mind bittersweet endings, it was still kinda abrupt. I mean, there just seems to be a significant disconnect from the rest of the story; a more well rounded reason as to why the ending had to be the way it was might've vastly improved the tale as a whole.
The narrator, who is having a great impact in the story, seems rather poorly characterized; I was half hoping he would slam the door shut in Celestia's face when she showed up in the end, if only because it would present us with more than 'nice guy who rears ponies'. Yeah, he has a tragic backstory, but the fact is that if we are intended to see this guy as a parent, you better portray that mofo like he's trying to protect his baby, and not as some half-assed brony. Which leads me to another point: having MLP: FIM as your reason for living is somewhat melodramatic, dude.
The story doesn't go for the easy cop-out of 'every human is a bastard', but I think having more than two individuals would've made this story a touch more engaging; and the whole mental reset button could really strike home how parting could've affected the world, in some small way (I can understand, however, why the author might not have done this; the length of the story would've magnified by a factor of ten).
The sad truth of the matter is that the only real reason this fic is so popular is because it appeals to the part of us that enjoys WAFF.
The thing is, though, is that I actually enjoy this damned thing. Call it a guilty pleasure, but what the hell...
I didn't actually cry, but it made me feel a bit sad and happy at the same time. And the fact of the matter is this; while I don't think this is the greatest bit of literature ever made by a fanfic writer, ever, the writer managed to make me feel something; it's quite an accomplishment, considering how frequently I can see some fiction and not give a crap about the cast.
And you know what? So what if there's a bit of Narm. There's a reason why there's another trope called Narm Charm.