Editing features and the ability to delete articles is a must, I'd say. Especially an edit feature. I wouldn't be too bothered about low PR and low traffic either, I'd be more concerned about the quality of traffic.
No point 1,000 people reading your article if only one converts. Well there would be a point to that, of course, but I'd much prefer 100 from 1000 converting to use an example, wouldn't you!?
Some sites I submit articles to don't have high PR's or brag about how much traffic they get, but they do send you targeted and quality traffic where as some high PR sites bragging of X number of visitors, don't convert so well.
In fact I tend to find the not so well known sites to be better in that respect.
If a big site is adding thousands of articles per day, then that will always decrease your chance of being seen and read by as many as possible as your article can become lost in a whole plethora of similar articles.
Also, many take to these article submission sites in the hope of boosting their own link pop and thus results from the serps, but what use is useless traffic? You want traffic that converts first and foremost, targeted quality traffic.
Anyway, I think quality is better than quantity and that should hold more importance than anything else really.
That and an edit feature ;)