I really liked the music and thought that it suited the mood of the game really well. You might want to fix the loop of the track on the main menu though: there is a click as the track finishes and loops back to the start. I've found that, if a wav loops seamlessly, compressing to an mp3 can still cause problems (clicks or other audio glitches as the music loops back around) but ogg files tend to keep a seamless loop if you achieve one with a wav. Worth looking into for polish week? I'd also consider adding in sfx for different interactions (jumping, reaching the portal, interacting with the user interface and menu buttons etc) and it would be great if the character disappeared as it reaches the portals, too.
I actually found the gameplay to be one of the more interesting aspects of the game. Like you say, there is probably lots of possibility to add complexity and interest in the level design, and perhaps to introduce additional mechanics if you develop this further.
Thanks for sharing, I enjoyed playing this :)