Aside from the mood (which I liked a lot,) I think having a swing rhythm would have made this a lot better. Something about the mixing also sounds a little weird to me - it sounds like your backing track had the highs turned down, and while I get that this can sometimes be a nice aesthetic choice, in this particular song it just didn't sound good to me. I also think faster decay on the notes would have made the track sound quieter and contributed to a calming feeling.