the 'feature' option features the story, which means it is put onto the front page as one of the three items in the middle at the top. That way it will show up better, so for samotnaf's blog for example, it would be worth featuring them, but for some of the other blogs, say the film review one, they would still have a front page link under the blogs heading but they wouldn't be featured.
If you want to add a tag then you just type it into the 'tags' box. In general contributors are encouraged to use existing tags as otherwise they become unwieldy and much less useful (in the past I've spent hours cleaning up the tags and others have spent even more time on it.). Just type in Free Flowing in the authors, people and groups tag box and it will create the new tag when you publish the article. Then all you need to do is open the tag through the link in the article and edit it as appropriate.
Posting to twitter is just automatically putting a the title and a link to the new article onto the libcom twitter feed so you might as well do that.
Hope that helps.