From an SEO point of view, I completely agree with solost - using subfolders like techinch.com/apps is much stronger than subdirectories or individual sites - especially if the topics are all relatively connected.

I'm not entirely sure how easy it is to do and it would take a bit of hacking with your Wordpress template, but as a compromise you could look at having each category page (so /apps, /widgets, /whatever) have it's own background image - although I imagine completely different themes or very different styles could be a bit of a nightmare.