Good concept.
I like this game, though it definately could have done with a bit more fine tuning; quite a few bugs in there, I think. To be honest I can't be bothered to list them because they all seem pretty obvious to me. Fix those up and you'd have a solid structure for future games.
In my opinion, "It" spawning on a line isn't a problem, I actually think that adds to it. Some might argue that the cross just packs a bit too much punch... maybe I'd agree with them, but I don't think negative scores are much of a problem because some people might want to see how low they can score and such.
Difficulty levels are good, music is good in my opinion (as in, it doesn't get annoying too soon). This just looks really promising. I can't wait to see what you bring out in the future, if you decide to build on this. You've probably got a lot to learn about actionscript, but I hope you find the motivation to keep at it.