The controls felt nice, but the game was lacking some polish. There wasn't an executable for OS X, so I opened "Reflection Runner Level 1.unity". I'm assuming that's the full game, and that's what I'm basing my review on.

1. There wasn't any sound. Sound is pretty important to the polish of a game, even if it's a simple 8-bit sound effect you generated on BFXR.

2. There wasn't a menu or (more importantly) a death screen. The player just died and the screen continued to move on. You can get away without a menu, but some sort of intro would have been nice.

3. The control scheme was a little odd. OP and space are all on different parts of the keyboard. Maybe use the arrow keys for jumping/sliding and space for going through a portal?

4. The portals didn't affect the gameplay. It was just a mirror image.

I liked the animation for the player, and the movement didn't feel floaty at all (which is one of the biggest issues I find with Unity games. People leave the defaults). So good job with that. A little polish and tweaking, and I could see this being a really fun game :)

The concept is nice, but the game should continue after you die, and not shutdown unexpectedly. Also, the down side of the screen could be different than the upside.