Some WordPress Blog Pages De-Indexed. Why?

Just curious about the nature of Google. I started a WordPress Blog aiming around low niche key phrases. I would get indexed within 15min as soon as I published one. As soon as I saw it indexed, I would do a search with that key phrase and check to see what the SERP was. It was usually on page 1 or 2. Then a day or two later, it seems that google de-indexed the page.

Each page was typed by me and wasn't really spammy at all. I typed each page from my head, so the content wasn't duplicated. I have my title as my key phrase, keyword meta tag as my key phrase, description as my key phrase but with other words to form an actual sentence, and then I have my tag as my key phrase. In the actualy body of the blog, I would have my key phrase in the H3 tag first, and then type out the blog. I would stategically place my key phrase in the blog and bold it some times.

What happened here?

Also, what's the rule regarding duplicate content? I have the All in One SEO plugin and have the no-index checked for archives, category, and tags. I have my blog's home page as a static page so duplicate content can't be seen that way either.

Finally, just a quick question. One blog that I did type received like 33 unique visitors, but I don't see any corresponding referral link or search term that could be associated with it. I hightly doubt anyone directly typed that into the browser. How did this happen?

Any input? I'm guessing it would have to do with the Google Sandbox. The site has been indexed in Google for a long time though, maybe 1.5 years. The only difference is I just recently setup WordPress on my server and started blogging <= 5 blogs per day for low niche SEO terms.

Tags shoudlbnt be no index.

Isn't duplicate content if 2 URL's point to the same text?