It's pretty much whatever comes in front of you from day to day that you are either fascinated with, excited by or knowledgeable about. Preferably all three. You can't worry about the rehash, because chances are extremely high that someone, somewhere, has already written about whatever you're writing about. My most recent post was about why do log files fill up. Believe me, other people have posted about it. But it's still one of the most frequent questions we see on the forums. You could write about it tomorrow and help three or four people that haven't seen the 7,432 other posts by other bloggers on the exact same topic.