I haven't started a blog yet, but something that is making me to create one this summer is simple:

Things that I can't find on Google.

Content on the web is spread across so many terrible websites with a lot of poor descriptions, and a lot of the time you can't even find the answer you're looking for. I want to start answering those questions.

Agreed. And not just code. Things you've done, why and how. Sometimes you spend a lot of time searching Google and Stack Exchange to piece together lots of fragments of information. Writing a blog post about it not only helps keep it in one place for yourself if you ever need to refer to it again, if you document the pitfalls you encountered it may even be useful to someone else.

This is a great way to contribute back to open source. Document work-arounds and how-to's and share them back to the userbase.