Rants are always an easy start. People constantly bitch about everything.
Or you can do the Seinfeld approach and look at some common behaviors and point out how ridiculous they are.
Politics - what's going on in a state, in a country, in the world
Science - the miracle pill that quenches your thirst (take it with 7 glasses of water)
Reviews - software, games, books, movies or other products
Educational - Stuff that your are expert about and what to share
Inspirational - stuff that fascianate you explain what and why or discuss a topic of interest
There's so many things you can write about. The purpose can be to entertain, to inform, to generate discussions/debates in the forum. It's not because it's on the geekery that it has to be geeky. Write about anything and everything.
Things that are taboo? Nothing really. Just remember there's kids looking at these pages so don't go sticking pr0n pics and stuff of that nature on the main page. Other than that, you have carte blanche.