I'd like to point out that I am rather conservative with my scoring so 8 is pretty high in my book.
The reasons for a score as high as that are as follows:
The gameplay is as promised and as expected. There is no real need to explain it more than hold left mouse button moves forward, release jumps.
The different enemies make the game much more interesting than it otherwise would be.
Certain aspects require a little thought. (I've been playing I Wanna Be The Guy too much... I was deathly scared of those spikes!! ~lol)
In the end, the main point for this game is that it is just short enough to be interesting all the way through. Anything longer (well, a level or two wouldn't spell doom, but 5 would be too many) would require more aspects to the gameplay, though obviously in keeping with the single button mechanic.