What it needs more than anything is rhythmic and melodic variation and stronger harmony.

The snare rhythm gets the piece going, but having it play what sounds like two measures on an endless loop grates on the ear rather quickly. At some point you should switch that rhythm between sections. Maybe let the strings play it marcato style for a few measures, then let the timpani have a go at it. And at some point, drop that rhythm entirely, either in favor of another driving beat or simply have the orchestra switch into a slightly more lyrical mode.

The melody can picked out with some effort, but it doesn't stand out as much as it should. Also try to incorporate several versions of the main melody. These could be simple variations in rhythm. Or you could keep the same rhythm but plug different notes into it. As an experiment, take your melody by itself and simply change the last note. Maybe a half step, maybe third, fourth, or fifth. Now listen to it. It wants to go somewhere else now, so just run with it and see where it takes you.

Harmonically, it's kind of a mess. Don't be afraid to explore different key areas. This keeps the piece sounding fresh, and it can help create clear divisions between different parts of the song. At some point, you should play the main melody (or its variation) in either a relative key or the dominant key to the original, then finish in your original key signature.

Here's some starter links. If you're going to write orchestral music, you should read up on harmony as much as you can. Then if you get a good grasp on that, start reading about orchestration.

(EDITED TO ADD: Actually, a good grounding in harmony will improve ANY kind of music you write, and not just the classical styled stuff. It doesn't matter whether you end up leaning towards rock, electronic, jazz, classical, etc. Western music has deep roots going back centuries, and being able to break it down into analytical structures will deepen your appreciation. Even if you end up writing unconventional or atonal music, you should at least be well versed enough in "The Rules" to know when and how you're breaking them.)

Key signatures
Harmony

There's also a boatload of instructional videos for music theory on Youtube. Also, are you in school? Consider taking an intro to music theory course to see if it sticks. I recommend the classroom setting for starters, because at least that way you have someone in a dedicated position of answering questions, and the other people in your class may ask questions that you wouldn't think of.

Well, there's nothing wrong with dissonance in music, in principle. I rather enjoy it if it's done well. But what your piece lacks is a feeling of strong movement *forward*. I think a better grasp of chords and part writing would help this.

You say that you have a basic understanding of harmony, yet you're unfamiliar with key signatures. Thing is, harmony is a much more advanced field of music theory than key signatures. Key signatures are one of the very first things you learn in music theory. And if you don't have a rock solid concept of how keys work, then any harmony lessons won't stick. It would be like someone who hasn't passed college algebra trying to do astrophysics. (OK, music theory isn't quite as dense as astrophysics, but the analogy still works.)

My advice is to pick up some books on basic music theory. Sometimes you can find great stuff in local used book stores, but the big box stores always have good stuff for beginners. And this is NOT a reflection on my estimation of your intelligence, but I thumbed through those "Complete Idiots" and "For Dummies" guides to music theory, and they were actually quite good. They had all the basic stuff you need as a springboard to start from, but they aren't too dense either.

Music theory can be insufferably cut and dried at first, but in any profession you have to pay your dues. Your music will improve the more you write, so keep writing! But if you don't understand your own music from a theoretical and technical point of view, then your music can take on the quality of a happy accident, and your capacity to make art *deliberately* will be elusive at best.

EDITED TO ADD: I almost forgot to mention it, but yes, some key signatures are harder or easier depending on what instrument you play. Any pianist worth their salt can plow through any key you put in front of them. However, woodwind and brass have a harder time playing keys with lots of sharps, and an easy time playing keys with flats. String instruments like the violin family (including the violin, viola, cello, and bass) have some difficulty with keys that have a lot of sharps OR flats. This is simply due to the mechanics of the instruments.

Learning to write for an orchestra is a long road, and one that never really ends. Don't expect an experience where the sky rips apart, angels sing, and the grand mystery of the Universe is suddenly revealed to you. However, there will be lots of seemingly insignificant "aha!" moments when things click in your head. It's the culmination of those tiny revelations that give you good experience to draw on as a composer. Just don't get discouraged. A bad day composing is better than an otherwise good day doing nothing. And just because there's a daunting amount of information to absorb doesn't mean that you have to get it all at once.

Start looking around the web for intros to harmony. In particular, find something that explains how SATB (Soprano Alto Tenor Bass) part writing works. If I find a good resource I may post it later.

While you're brushing up on basic music theory, start training your ears to listen for how things are orchestrated. There's a very good introduction to the subject I found on the website for the Garritan Personal Orchestra. That's the package I use for my sound library, and it comes bundled with a sequencer called Overture. It's a fairly cheap way to start learning, and I definitely recommend it. No, I don't work for them. :)

Intro to Orchestration

Check this out. It's a free online resource that's put together pretty well. Use the pulldown "Lessons" box for starters. After key signatures, intervals, and scales, it gets into the trickier bits like chords and analysis by Roman numerals. Understanding this kind of stuff gives you a better grasp of what's going on "under the hood", so to speak. This can then be applied to your own pieces.

Musictheory.net