I'll put most of my commentary here (as opposed to your other recent submission).
My first reaction to both pieces was, wow, these are well-arranged. Your choices for instrumentation are a little nonstandard (not that anyone just casually listening with a GM bank could tell), but you use each one appropriately, with varying degrees prominence and function. This is very good. This piece, for example, makes good use of a dominant pedal that never resolves to build tension, that's explicit in the piano but reinforced through your harmonic choices. Having said that, I think the driving percussion and general tone of this one is too "imminent" for any location music ... it's an odd mix of reflection and anticipation. Not sure where else it would fit.
My main gripe for both pieces is that they lack clear direction melodically, and to a lesser extent, harmonically (you at least manage to establish a clear harmonic progression here). My advice would be, when you next feel inspired to write, let your initial idea lead you either into patterns of harmonics, or into a melody that you can develop. Right now, it just seems like--even when you find a harmonic progression you like--you don't go much further and simply improvise other parts over it. This isn't always bad, but it steals coherence from the overall work.
Hopefully those comments will help, and realize I'm being pretty critical. My opinion is that your work shows a lot of promise; even right now, without any improvement, you'd probably be able to do an excellent job at arranging the thematic material that others (including myself) are working on if you aren't feeling particularly inspired yourself. The project is a collaborative effort, so don't be afraid to "hitchhike" off of others' work.
Now, about MIDI: I personally prefer to see this format being used for submissions. I want to be clear that contributors should also be reviewing the music, not just how it happens to sound (though execution is an important part of the final product). MIDI is excellent, because I can go back and look to see what exact notes you decided to use, or see what it might sound like with a piano instead of a guitar, etc. The problem, as Temp said, is that the sample quality varies from system to system. Unfortunately, converting MIDI into a wav format is not particularly easy if you don't have the hardware to do it. There are also several software solutions to try, but in my opinion, none of them do a particularly good job of it. I'm fine with doing most of this myself, in the end; it might be a good idea to keep the "feel" of the music generally consistent anyway... but this is another topic I'll bring up elsewhere at some point. Bottom line: MIDI is perfectly acceptable.