I believe the main page should be able to showcase as much stuff as possible whilst maintaining some sort of order so it can look good. It also must have the power to push the visitor into getting addicted to participating to it.
Your main page should show the most complete topics - those who are full of activity, those who get the most views, those who are the most controversial, those with the most information ( pictures, details )... Pretty much like Wikipedia's featured article which always feature a very good page that says "this website is serious".
We need to know your website can potentially be about anything and the first time visitor needs to be aware of this ASAP if you want him to stay, so we should be able to see as much stuff as possible without scrolling down. Perhaps some sort of category "wheel" could be made, although I don't even know how to make it. Something that would include all the main categories, such as science, sports, etc.. And whenever you'd click on one, you'd get sub-categories. Something similar to the current pie graph could be good enough for me. You'd just have to make it interactive so we can "click" on the pie, you know what I mean?
Polls are what brought me to dig deeper in your website - then I began writing stuff. It's really fun to vote and compare with what people chose. It's the best part of OpiWiki in my opinion. Maybe some sort of neverending list of polls "app" could be made as well. Example: you vote for something, then you are instantly being transfered to another poll and so on - and if you don't want to vote, you just "pass" to the next one, yet always staying on the main page.