I feel the way reddit is made with the downvote and upvote buttons are really the best way to filter and promote stories and I'm not really a fan of when functionality is taken away from the subreddits voting system.
I feel the downvote button helps keep away spam, nonsense, and just gives more community control over the subreddit, which I feel is really the *essence* of reddit in the first place.
Also, I know that you can disable the CSS with RES, but I don't really use RES, and I quite like the style besides the lack of a downvote button, so I really don't want to be switching back and forth to vote on things.
Also, if [you want to be a little cheeky about it, I have a rough idea/template for what the downvote button could be.](http://i.imgur.com/iKtLX.jpg)