In my humble opinion,
First of all, it seems that many english speakers have not found the game’s story to be as compelling as the rest of the game itself (and I include myself in that batch). Recently I’ve come across an insightful argument that caught my attention. The writer argued that story length was too long/expansive for the game's twenty or so chapters. I believe this is precisely the problem with the game. There are too many arcs and some that do not need to be there and still some that are just not believable like the old classics. A Prince and a Princess are being escorted by ONE meagre Knight across the country? Come on! What is that? The logic of that action given the current political system of the time just doesn’t make sense. What if Chrom and Lissa found the avatar floating down river to the banks of their garden inside the castle? Yes, that’s is just as convenient but the difference here is that it is much more believable! Details such as these, if they were given a little bit more thought, would have resulted in a much more engaging story.
Although, on another note, a more positive one this time; allowing the player to insert themselves into the story was a very clever move on IS’s behalf. A tactician that actually helps out. Who knew right? Lastly, one of the other elements that I believe contribute to a sub-par story is that it’s most fragile component - the dialogue itself - was perhaps damaged in translation when it set out overseas. I sincerely believe (and hope that it is true) that the Japanese versions of the characters were much more fleshed out, their dialogue much more attuned to their personalities than the one’s we’ve received on our end (I am looking at you Say’ri and Chrom).
Other than that, I do not think anyway who has been paying attention to the story and have completed the game can say that it was anything other than that they enjoyed the journey of the Chrom, Lissa, Fredrick and ofcourse me, Primrose!