"What do you mean it's not finished yet?"
Now that you've got a concept and some kind of a rough draft done, it's time to get an outside opinion. If you have any friends (which I hope you do), ask them. Preferably, ask non-scene friends. Tell them to find something in each part that could be improved. The less they know about scene and computers, the better, because they can be more objective. Ask non-geeky people. Ask girls on your class. If you still live at home, ask your mother. Your mother will know. Then think of what they've suggested and implement it if it's feasible.
If you've got a good graphician, give him a complete control over the entire demo. Or at least, give him the veto-right to the design decision you do. You'll be better off, trust me. He's the graphician because he knows what looks good, he's not just a texture-factory or the person who'll fill out the missing fifteen seconds in the demo. And if he's just a friend and not really good at drawing or making textures, go find someone who is. It will pay off.