I have just played through the levels. :) I have played some other puzzle games which had comparable principles, but each one has its own  differences.

Some comments:

1. I like the retro style; and the Super Mario Bros.-styled coin sound for validation! X)

2. At the beginning, I was wondering whether I was presented with a loading screen or other; it took me a few seconds to realize the game had actually begun! This is because you have chosen a top view, which is visually nice too, but hides the cube’s geometry.

3. Picking up on this point, I think it would help the player to show the state of the other faces. I have seen a handful of other puzzle games using a die or several dice, and I realize they were all in a (possibly pseudo-)3D perspective. So, a solution is either to switch to some pseudo-3D, or (if you want to keep your own 2D top view visual style, which I recommend, as it makes your game different! :)) use some visual indicator. (This is even truer for those games that use non-standard dice that do not allow you to deduce the opposite side of a given face.) (For example, I put a UI element in mine. Well, I did not make it in time to display the faces on the actual die, so this idea saved my life since I set it up in place early! X))
   

   Of course, that depends on the kind of difficulty you want; if it was your intention all along to have him use his short-term memory and spatial visualisation, then you are already all good. ;)

4. I think you should make it clear to the player that he has completed the last level, just to be sure.

5. You used the arrow keys, and so did not run into the now famous (well, for me, at least XD) QWERTY layout problem. ;)

As you say yourself, there is essentially one mechanic and few levels (but that is fitting; no need to drag on if you feel that would be superfluous). I think your game is simpler than some comparable others, but you did a fine and neat job, with reasonable difficulty, and your few levels are correctly designed (contrary to some other people that may have rushed some of their levels… :o *looks sideways*).

So, I think it’s a fair one. I cannot give you the originality distinction in a context of comparison, as other people also thought of the idea and sometimes took it further, but I like the game and its own Mario-like 2D style (I think you’ve had me with this, nicely done XD). :)

Nice job.  I saw a few other games with the same game concept but I loved your graphics.  Nice job

It was fun

Basic but fun, nice simplistic art too

I like this concept! i think it's one of the most creative concepts i've seen thus far. personally didn't get very far since im bad at spacial remembering but i really liked it!