Can someone explain (using very simple words) the Slashdot concept?
It's not a simple system.
But generally, comments can have a score, between 0 and 5 (IIRC - might be -1 and 5). Comments start out on score 1 (if the author has a bad reputation it starts at 0, good reputation it starts at 2).
In your settings you can set the minimum rating a post needs to have before you see it. I think the default is 1.
Every now and then, users get "mod points" that they can use; say you get 5 mod points, once per month. You can use these to give +1 or -1 to posts you choose (not your own).
The actual system is much more complicated than this.
A common complaint is that there is a lot of "groupthink" -- if your opinion isn't the common Slashdot opinion, it's likely to be modded down. Not everybody agrees this is happening.
Odd, let me try again.
I believe that Slashdot has a (large) number of "moderators", each of whom gets an allotment of votes each week with which they can vote individual posts up or down. Users can then, based on these rankings, filter out any posts that have been voted below a particular threshold allowing the "cream to rise to the top". Unlike the current super-"moderators" here that can actually edit and delete forum content, this is their only additional privilege so it can be disseminated quite widely, possibly based on an activity level threshold? (I'm not sure how it's actually done).
Since the ratings live and die with each individual post there's no user level "reputation" ranking that's retained -- each vote is ranked on its own merit. I believe that this was your biggest concern Batgirl, was it not?
I'm also of the opinion that the voting need not be limited to just a smaller group of moderators. It seems to me that this system would be just as effective, possible more-so even, if everyone could cast their votes on individual posts.