Being the founding developer of a neat start up, your experiences about the process up to now. Layman level discussions of your algorithmic trading approach / data mining adventures. A combination between layman and geeky stuff would be different.

Whatever you're passionate about!

Sound advice. The only problem is that I'm so passionate about this things that I am passionate about I find it hard to stop and write about them :)

Ha nice but you sure as hell shouldn't stop to write about shit you aren't passionate about!

This much we can definitely agree upon.