This is my first critique, so I hope it goes well.
The background definitely fits the theme of the photo well. So does the hairstyle, outfit, and colours of the figure. You did a good job on the snow and the decorations as well. The smaller transparent figure in the bottom left corner is a nice touch, as well. I really don't see anything wrong with this photo, but it might have looked a little better if there was more snow. The tree on the right could look better with some decoration, too, but as I already said, the photo looks great the way it is. You did a really good job on this.
This really is just too adorable!!!! I love Hatsune Miku to death, and I really can't wait for Christmas! So this is a perfect combination of all the things I love I really love how Miku isn't in her normal clothes, but in such a cute little coat! And the bow on her head adds a cute touch Then again, she would probably freeze to death in her normal clothes, lol XD The tree is cute, I think it would add a little more flair if it were decorated, though. It still looks so perfect as an entire picture. Keep it up!
The colors are really awesome. Snow Miku is too damn cute. The "merry christmas" is too damn cute too. ^.^ The pictures is full of details, with the "frozen miku" :3 and the fake snow.
And don't try to understand why it's a totall success, sometimes it can be explained. But well in my opinion I would just say that it's because it's really a awesome picture and you put a lot of efforts into the details.