I like your stuff so far, and this is no exception. The game is difficult, of course, but have you really seen any of the top quality side scrolling shooters that WEREN'T difficult?
There isn't much I can say about this that I wouldn't say about other side scrollers, but the leveling system and switchable/purchasable weapons make for a nice touch. Having to rely on picking them up in the middle of a heated battle would have made things a lot more chaotic and unpleasant. And of course, you're pretty much dead honest on the concept that these games are solely based upon rampant destruction and death for the sake of winning the game while the loser has to bite the dust.
The only niggles I have is that the flame weapons make it hard to see where enemy projectiles are. If the attacks had more solid shapes, it would make it a whole lot easier to see them and know where to dodge. I think that might make somewhat of a difference in how long people last against bosses.