Before you can answer this question, you want to be clear on what is the purpose of you writing
your articles in the first place. So what I will do is cover a few different reasons and answer the
question based on those reasons.

1) To get more recognition in your field and position yourself as an expert.
If this is your main purpose, then you want to submit your articles to established websites, e-zines, magazines, newspapers and trade publications in your field. You will want to subscribe to these and look at a few issues to see what kind of articles they accept and the writing style of the articles they generally accept. When submitting to these areas, it is very important that your writing be very impeccable and clear. Your purpose is to sound like an expert and the only way you are going to show that is through your article, so if it sounds like it was written by someone who doesn't have command of the English language and is full of errors, you'll look worse than an amateur...... that is if your article even gets published in the first place.

2) To get a butt-load of traffic to your website.
If the purpose is to get a ton of traffic to your website, then you will want to focus your article submission to high traffic sites that are ranked in the search engines. This doesn't have to be article submission sites exclusively, but definitely sites that accept articles. You want sites to have the following characteristics.
1) An RSS feed of the articles that are posted to their site. Even better is an RSS feed of each category on the site.
2) A high Alexa ranking (in the top 100,000)
3) You can find your article within 2 of clicks when newly submitted. If it is to an article submission site, most decent ones list the newly submitted articles on the front page.
4) The site makes your bio links active so people can go directly to your website.
5) There is some sort of approval process so the site has some sort of quality control. If people see the site as spammy, they won't bother to even read the articles on the website.
6) The website makes it easy for others to publish your articles to their website and newsletter (provided all links remain active).
7) Not the most important, but it helps if the website features articles. If it does, ask them what they look for in a featured article.

3)To build up link popularity and authority.
What you want to look for here is established web sites that have a lot of their search pages indexed in the search engines. The strategy here will be a bit different. In this case, you want to look for sites with the following characteristics.
1) The site has some page rank (at least 2 or better) and seems to be growing on a consistent basis.
2) The site does a direct link to you and doesn't do "nofollow" link attributes or redirects.
3) The site allows you to put your own anchor text in the link to your website
4) The site doesn't require you to link back to them - this is a nice have, but I wouldn't make it a deal breaker
5) The site allows you to to put more than 1 link in your article or resource box - again, this is nice, but not another deal breaker.
6) The site is closely related to your niche. The closer, the better.

These are just some of the things you want to look for when looking for sites to submit your articles to.

Keep the questions coming!