When Slashdot added the journals, one of the features was the ability to submit journal entries for stories.
But other than that, I've never read any guidelines for what goes in your journal. It's up to you what story you want to tell in your journal.
- You could complain about work.
- You could post about exciting programming challenges.
- You could talk about your personal life.
- You could talk about the challenges of starting a new business.
Really, you could talk about just about anything that isn't actually degrading and insulting to others. (Not to say that there won't be chip-on-the-shoulder types who look for something insulting in what others say, so you will offend people unless you say nothing of interest.)
The key to me is a journal like a business plan: It should be about you. It's your story.
Tell it to the best of your ability, and if you can, give people a laugh along the way. Everyone needs a daily giggle to keep their sanity.