Main riff was pretty boring, and deffinetly needs variations and such. That's for starters
Id have a second guitar part running in harmony with what yo uhave their already, something building up aybe so when it drops its got a climactic finish. As Behzad says ur ain low end riffing is a little uninspiring, i think you could probably add some more flair to that with your rythm.

Realy nice melody you have writte there though, you have some good ideas, just try and augment them with more ideas. It has an Iron Maiden feel to it so far so maybe look to them for some inspiration?
I reiterate what the others have said. I like the climax of the song, though overall it's a bit mellow. But I think with the drums and other instruments and/or effects, however there would be a different feel to it, sounding much more awesome. Keep on working on it.
Fill the void *bump bump bump*