Any given comment on Slashdot has as special ID number called a "cid", which (logically) would probably stand for "comment i.d." Each post, right after the timestamp, has a link saying something like (#CID) which will take you directly to the URL of that particular comment, and the URL of any particular comment ends with "&cid=" followed by the CID number. Cids are not handed out linearly-- slashdot kind of shuffles the cids in a funny random way, such that if you post a comment your comment will be given the first available cid plus some random number probably between one and twenty. The reason for this was to defeat the first post people, and to an extent it worked; it didn't make the first post people go away, but it took away their satisfaction level by making them know that they would be almost certain to get some random cid around five or six and thus not actually get the first post in sorting even if they got the first post chronologically.

The importance of the cid rests in that pretty much everyone on slashdot reads the stories sorted by "oldest first", and pretty much nobody on slashdot has a long attention span. Almost nobody reads slashdot stories more than a day old. Almost nobody, after reading a slashdot story, will come back later to see if any new insightful comments have been added. And most importantly: Almost nobody on slashdot reads comments with a cid of over 150 or so. Rather they will read up to around 150, lose interest, and wander off. So no matter how insightful or creative your post is, no matter how pointless and redundant the first 150 posts were, if your post is around 150, almost nobody will moderate it up or even read it. And if your post is up around 300 or so, or God forbid 600, you might as well have not posted.

And this just leads to all kinds of nasty problems. Knowing that the secret to being read, being moderated, and getting karma is not in writing a good post, but in writing a clearly visible, attention-grabbing post very early, people act accordingly. People will constantly reload the slashdot page, hoping to come across a story just posted in hopes of being able to grab the one post in a story that first states the obvious and gets voted up to score:5. People will rush themselves, ignoring whether they should think harder or write more within this post in favor of just getting something on the page before they're scrolled off the bottom. People will post without reading the comments already there, since reading comments takes precious time, and thus inadvertantly post comments which are wholly redundant. Even those who care less about karma than about writing good posts are affected; they look at the story, read the comments already there, and know they could write an insightful post, but do not bother writing said post since they know that by the time the comment is finished it will get cid 250 or so, and nobody will read it. Meaning that you have a vicious cycle; at the point at which the story has 150 comments or so, people stop bothering to try to post insightful stuff, but the 150 comments already posted are all mediocre rushjobs.

See also: uid, sid, people who post on slashdot without first reading all current posts in the story, three dollar crack.