There are a lot of posts in the Blogosphere about how to choose a topic to post about, where to find content, how to link to good resources.
Even though one of the most commonly cited fears of newbie bloggers and newsletter publishers is "What will I write about?" — I believe this isn't the real problem.
Once you get the hang of blogging, and you've developed your posting wings (2-3 times a week), the real problem is filtering out an excess of information. You must narrow down your focus. Weed out all the dead rabbit trails.
This is the real problem for most of us. I call it Reverse Writer's Block. It's when you have so many ideas and resources and things you could write about, you grind to a full halt.
The real question becomes: what business am I in? What am I in the business of selling? What do I want readers to spend money on?
I'm not denying that writing clever blog posts and newsletters that showcase your expertise isn't foremost. Nor do I diminish the quality of relationships intended in your content. But if you're in business, I'll assume that you have something to sell.