Don't feel bad, making Magic cards is hard. You need to have a pretty good understanding of the game's past and present to really nail down quality design.
My first point is something you pick on if you've been playing for a bit. I stress/bolded my point because in the submission article they explicitly tell you not to submit enchantments that tap. So I'm worried the card may be thrown out as soon as someone in WOTC sees you didn't read the instructions.
The second point comes from the fact that, as you correctly identified, the "Fling" ability is primarily in Red. Especially the artifact part; both Mirrodin blocks have set a precedent for Red to sacrifice artifacts to deal damage but not Black. Black can sac things to deal damage but its much less frequent. Furthermore, almost all Black direct damage, of which there is much less than that of Red, it's flavored as draining (X damage, gain X life).
