Good shot!
It was a nice shot! There's definately potential in this piece. Let's see if I can throw some pointers your way. There needs to be some more instrumentation and a bit smoother riff work. I know this is midi, so there's limitations there, but you can still score more to it. Add in a bass or something to fill it out, I see the idea of a single guitar in the beginning, just remember when working with midi files it has a very bland electric sound to it so the more sound to fill with is always good. I like when the other guitars come in, that adds a nice fullness to the song. It's a good melody, it just needs to have that 'umph' for a heavy metal song. Keep workin' at it! Also, a suggestion from me would be to try out somethin' like Fruity Loops, just so you can import the midi and see what it sounds like with different soundfonts and what-not. Keep at it!
~Dylan ^_^