1) The chords are simple, but effective.
2) The instrumentation is well-intended, from what I see.
3) The melody and how everything is layered together work well as a sinister-sounding theme, from possibly Portal, if given more gravitas.
1) Not enough reverberation. The instruments sound very MIDI-ish, and fake. There's also another contributing factor to this which I will get into very shortly, which is...
2) INDIVIDUAL INSTRUMENT VOLUME. Now I have been guilty of committing this same mistake in my earliest works, but so have many others. There is next to no difference in individual note volumes for each instrument. It doesn't sound like there's the touch of the Master's hand. Close, but not quite there, because it doesn't sound like it's being *played*, rather than sequenced.
3) The piece itself doesn't have enough gravitas. A little slower, a little more sombre, and possibly a bit more instrument layering? At the moment, I know what you're trying to convey, but you're not quite there yet.
4) Looping. What's with the abrupt ending? This piece would have worked beautifully as a short loop. But if you want people to have a long, enjoyable listen, go past the A minor and G minor bits, and introduce new themes, but along the same sinister lines.
tl;dr: I quite like this -- and it's crying out with potential written all over it -- but lengthen this, build up on your production skills, and give the piece a bit more gravitas.