I can echo Cedric H. here. I never actually understood the system and only just found out about the score being displayed on the green bar. I did not know that was there!
Completely! And the fade from dark green to light green. I always thought that it was building up to a yellow or white as 'complete'.
To the question though, I do enjoy the rep button and use it as a like button, like others mentioned above (although I may use it a bit too frequently sometimes!). Evening out the scores being awarded is a good idea though, particularly as time progresses this will only get more out of sink. I do not think negative rep is a good idea, again for the reasons entered above about it not being constructive. Rep should be more of a compliment in a my sense.