The good is you NAILED it. This feels for all the world like a major-studio release from the Genesis era. Visuals, sound, gameplay, character physics, power-ups, everything. It's solid, it rewards exploration, it's got crisp visual design. It's clearly a labor of love and it pays off.
But I have to quibble, and this is where it loses the half-star. Five levels in and I haven't really found much that varies up the gameplay as you progress. The bouncy timed bonus rescue levels are all well and good, but the normal levels all play very samey, despite the change in the (gorgeous) visuals. Yeah, the games of the old era did that too, but, well... it's not really enticing me into sticking with the game to its end, especially as the levels get bigger and bigger.
Maybe if you do a sequel, change the weapon power-up for various levels. It could create some new exploration puzzles based on the different weapon dynamics.
Overall, a damn fine game you've created. Tweak the sameness factor and it'd be just about perfect.