I have an impression I've talked about that already, but can't find it.
Why are headers so huge and contain so much information? This is probably useful when topcis only view is used. Maybe some information can be hidden when discussion is opened? I'd say that actually none of that information (i.e. anything but topic) is needed when discussion is opened. You can reply below and see activity yourself.
Also it would be nice to drop background color for the headers (that would also remove the need for padding and make them smaller). Whitespace is enough to separate topics (or at least I hope so).
Please take a look at http://practicaltypography.com/websites.html Maybe some of that could be incorporated into general design guidelines?