New boards are added/moved around/(Have there been ANY boards deleted??) as the administrators see the need to help move the site along better. For instance, the large amount of posts about DVDs impressed upon Brian to create the new DVD page. And off topic posts about the boards in general prompted them to create this page. Having new boards where people can post about subjects in a more general manner helps the other boards to move along better, and not to be cluttered with off topic subjects that would be better suited on a new board.
And as for the desciptions changing, this helps newbies to figure out what boards they would like to visit. And, yes, I did notice changes like this from time to time on the old archive boards.
As for your deleting threads, that can only be performed by a moderator. So if you want a thread deleted, just mention it in the post, or send a message to a moderator to have it done for you.