Better than a sharp stick in the eye...
Wonderfully executed.
Two things I would like to see. The first would be a scenery change between levels. That alone will kill 30% of the monotony factor. The second would be more than bidirectional targeting. Side-scrollers like this have a tendency to send enemies from above and below. I want to slaughter these things, not hide from them. I don't have any Flash programming experience, so I don't know how difficult it would be to code, but I would definitely work that out.
Most importantly, I absolutely love the fact that when you die, that's it. Keep doing what you're doing, cause it's working.