Seems more like a work in progress
I have a number of issues that I would like the author to address before it is considered a finished product.
1) The only reason I found the game challenging was that the controls were awkward. Many emulators or other games use the spacebar or "X" as the firing mechanism and "Z" is usually used as a cancelling or "go back" button. I would suggest changing the buttons to make them more familiar to your audience
2) Plan out your level maps better. Metroid had variety in even the shape of the rooms you would explore. I would imagine many people would find it dull just to play through a grid.
3) I would suggest making enemy collision boxes larger and have them move toward the player, as opposed to having a simple loop. This makes gameplay more challenging. Otherwise, players might experience the same problem as I did which was walking through much of your game without having to engage any of the enemies.
4) Take more care with your graphics. I understand that you are only making a Flash game, but even in the NES version of Metroid, the graphics were incredibly detailed and added to the atmosphere of the game. I found that much of the time, your graphics were distracting in their mediocrity. Take time to create your own style, as opposed to doing a faux-anime ripoff. Gradients should not be your solution to everything. In addition, the whole screen looks flat and if you took some additional time to add in highlights and shadows, I think that you'll find it gives the graphics a lot more depth and makes the game more inviting to play.
I'm not sure it was intentional for the mouths not to sync up with the voice-acting, but that was also a detail I would correct.
5)Please, please, please add a mute button. I know you're probably trying to save on file size, but just looping the same 10 seconds over and over is downright infuriating.
That's all I can think of at the moment. Please don't take this as a personal affront and accept this solely as constructive criticism.