First of all, you were flat a lot of the time. It sounded like you weren't really comfortable with the song, or that you knew it very well, as the melody of the song kept changing quite frequently - almost as if you were making it up during the parts you forgot. I know how exciting it can be to want to get a song out, but trust me, it's best to wait if you're not entirely sure how the song goes. ^.^; Not having a great grasp of the melody causes you to become sharp or flat in places, as I'm sure you know.
In addition to this, you sounded flat because you were restricting the air support by not opening your mouth wide, in the shape of an "O." This restricts the diaphragm and makes some of the more powerful notes sound nasal and/or whiny. You don't want that. So always be sure to open your mouth nice and wide, and make sure to give yourself enough support by taking nice breaths through your diaphragm. Which brings me to my next point: Sing through your diaphragm and not your throat. As I mentioned, using the diaphragm helps a lot, especially with giving support to those more powerful notes in the chorus, and will help give you a nice, rich sound throughout the piece.
Secondly, a bit of your pronunciation was a bit off. It might have been the timing, or it might have been because some Japanese words are damn hard to say, but.. some of the words were noticeably mispronounced. This isn't so much of a big problem, I just happen to be a huge fan of the song. >D;;;
Thirdly, while you were in tune for certain parts of the song [especially the harmonized places in the middle of the verses], you fell apart a bit when it came to some phrases. For example, "yasashisa ni motare~ amaeteta yo ne~" [the parts with the "~" indicate holding the note and putting a light crescendo on it.], sounded lovely leading up to those ~notes, but once you reached them, you totally held back and made yourself lose power. And this was especially noticeable in the chorus parts, as you put a bit of a heavier crescendo on the lyrics there. Don't hold back, that will just lower the quality of your singing. And I know you're too good a singer for that.
Again, these things are just things I thought could stand to be pointed out. Not trying to be harsh. ._. Just trying to be helpful. I know you have a lovely singing ability, I think this song, though, wasn't entirely suited for your voice. ^.^;; I would love to hear you sing one of Megumi Toyoguchi's songs.. like "Misty Eyed" or "Nemeru Omoi" or something. ^.^