In Digital Point, Matt Cutts explained saying:
"I'll try to give a few rules of thumb to think about when looking at a directory. When considering submitting to a directory, I'd ask questions like:
- Does the directory reject urls? If every url passes a review, the directory gets closer to just a list of links or a free-for-all link site.
- What is the quality of urls in the directory? Suppose a site rejects 25% of submissions, but the urls that are accepted/listed are still quite low-quality or spammy. That doesn't speak well to the quality of the directory.
- If there is a fee, what's the purpose of the fee? For a high-quality directory, the fee is primarily for the time/effort for someone to do a genuine evaluation of a url or site.
Those are a few factors I'd consider. If you put on your user hat and ask "Does this seem like a high-quality directory to me?" you can usually get a pretty good sense as well, or ask a few friends for their take on a particular directory."