TROUBL?
First of all, this is great! Don't let any of the criticism dissuade you from continuing to make games (or improvements to this concept).
However, there are some improvements to be made. First, you guessed it, number of keys. Eight sounds like a good number to me, at least for easy mode. I would also suggest moving the keys to the home row (asdf = left hand, jkl; = right). It just feels more comfortable to me (I'm sure others agree). Maybe even an interactive tutorial would help the learning curve a bit.
Song selection is going to be the other barrier between you and your audience. While I liked all the songs in the game, other players are going to be more discerning (read: picky). Personally, I'd make this game into a "Piano Hero" of sorts (as opposed to that popular rock guitar game) Theme the game with songs played on piano, organ, keyboards/synthesizer, or even accordeon.
I'm sure others have suggested artists to include, but keeping the above theme in mind here are some artists I'd like to suggest; Jerry Lee Lewis, Ben Folds, The Dresden Dolls, The Doors, Cat Power, Daft Punk, The Fiery Furnaces, Queen, Radiohead. Just to name a few XD. Really, all these artists play piano/synth and are fairly well known. Obviously all that is just a suggestion. If you want to take the game another direction obviously it's yours so... I just thought it'd be cool.
Finally, the in game backgrounds are a bit, umm... bland. Though I suppose they work well enough for the purpose (separating the notes from the background) the same could be accomplished with borders around the notes, freeing you to pursue more interesting backgrounds (nothing too busy though! The notes are complex enough!)
Whew! Sorry that was a bit long winded. Keep it up! I look foreward to seeing something outstanding from you soon. Just don't let people that say stuff like, "FAIL, dis is teh SUXORZ!!1" get you down.
Carni