For starters, I'll give you props for doing this in Garageband. I don't think I could make dubstep in that program (I use FL Studio for mine). But as for some constructive criticism: I think you need to take a second look at you EQing. I'm getting A LOT from the lows and the mids, but there really isn't much in the high ends. Your cymbals and hats need to sound super crisp up there, but they are sitting a bit too low in the mix I think. Also keep in mind that the bass, although it's a bass, needs to have a high end. It will give it grit and crispness and it will fill seem to fill out more space. Like the other reviews say, the elements are here. I hope I gave a few good suggestions of what to actually DO with those elements. I'm gonna follow you because I'm really interested to see where you're music goes and how you progress!
-CiMba
P.S. I could always use some feedback on my tracks if you ever have a minute! Thanks!