Beauty is in the eye of the beholder, as they say. First, different people have different views on what makes a "good" site. It is for this reason that we have the ODP guidelines and note that a lot of it is left up to editor discretion.
Second, standards change - years ago, when the ODP was a fledgling directory, the priority was to increase the size as quickly as possible. The focus has changed much more now onto the quality of the sites listed. So there may well be some sites listed which, if submitted today, would not be. In these sorts of cases, random reviews of categories by editors will remove those. If "worse" means "not meeting DMOZ guidelines", then you can use the abuse report procedures on DMOZ itself, or PM a meta as described in the abuse forum.
Third, sites change - what was once a great resource might well have been abandoned by its webmaster or changed to become something completely different. If you find such sites, and wish to help the ODP become a better directory, then please submit the category and site information in the Abuse forum - this will bring it to our attention.
Finally, as the ODP matures, the bar for "unique content" gets raised higher and higher. We don't look at submissions and think "Well, this is no worse than anything else so I'll add it." How does that improve the category? If you wish your site to be listed in a particular category, it should be better than the best site already listed and offer content not offered by any other site already in the category. Don't aim to be second worst, aim to be best.
