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I am to give a talk on wordpress.com to a beginner audience, and am searching for as many features of wordpress.com that are "important to share" with them.
So basic things like "writing a post" is not it. But things like "Tags in WordPress are shared by all. So use tags so people would find what you are writing" IS a good tip.
How about adding Gravatar to your post titles to highlight the comment author?
Making sure to get your Akismet account setup right away, or potentially use a system like Disqus to manage your comments. There are plenty of "war dialers" out there looking for fresh WordPress installs so they can start spamming your comments.
Related to this might be how to keep an eye out for spam comments that still make it through. The bots are getting more sophisticated in their wording to sound human, but there are still a few things (like checking the e-mail address used for the comment) that can indicate spam comments from real ones.
Changing your permalinks.
By default, the install uses a simple post Id for your posts, but it would work out better for SEO (not that it is a requirement) to use a different permalink, such as the day/name format.