They should have good PR and for sure a good article accepting policy.. The more quality an article directory mantains the more good it will be for you
i generally go for an article directory that has good home page PR, nice categorization, where article get cached easily, has static URL.
I think the best criteria is check the PR of the site. If the PR is good then the chances of article caching will be high as compared to low PR sites an Google will give value to the link given by you in the article.
A few things to consider while choosing article directories - Traffic penetration - Auto approved or moderated (Moderated directory is preferred) - Type of backlinks (do follow or nofollow, after Panda update most of the article directories are turning nofollow.)
I'm putting cents here: The best article directories I've ever had articles submitted to were MyArticleNetwork and Article Ranks. These two are by far the best I've ever seen in the industry, even better than Unique Article Wizard. I have used ArticlesBase very successfully for gaining backlinks and website traffic. GoArticles is another, but I they used to be better over a year ago.
This really depends on what your goals are. You can auto submit to 500 directories which auto approve but they won't hold as much weight as a buzzle.com article or even Ezine. So, are you trying to build quality looks or is this backfill to expand your anchor text targetting?
High PR, DO follow these are the most important parts and again another is categories. select the exact category.
Relevant to the theme of the site, good content, not allow spammy links and should be Nofollow to have any real worth at all.
The main step to keep in mind while article submission is that directory has the catagory according the website that you wanna submit.
AS specially do follow, good PR, different c class ip address and relevant category are required for choosing good article directories.
You should select the criteria based on reputation, so always submit to the likes of Ezinearticles and Goarticles
Check the PR must. If it has good PR then your article will cached easily. But, if it has low PR, then it may be your article will never cached.