Daniweb... ...The home page shows almost no content about the fold for a 1240 width screen. Not so when you reach the actual forum threads.
I'm not seeing much content above the fold on a 1024 x 1280 screen for the main forum pages either... at least not with a couple of toolbars and the browser window not completely full screen.
In the Daniweb "Windows Software Forum", eg, with just two large
ads at the top, I'm only seeing one forum thread peeking up above the fold... and the third thread is an ad disguised as a thread as well... and all are designed to pull my clicks away.
Beyond this, several factors...
There's a global mega-menu up at the top (with some nested menus as well) that's simply got to be hugely confusing to Google. There are also probably 100 tag cloud links at the bottom of the page, and for the most part they're pretty thin pages.
Even assuming that Google has implemented a Reasonable Surfer Model, with all those links distracting from the main content, the 30-40 content threads displayed probably don't get an adequate share of link-juice/PageRank, and it's probably impossible to prioritize link-juice distribution within the site.
Also, on the thread links there's a "feature" that I haven't seen on any other sites... If you mouse over the thread links, the title attribute displays a huge snippet from the first post of the thread. I could argue this two ways...
a) that it might help users by "previewing" the thread for them, without a click, thus improving the user experience...
b) or that by reducing click-throughs to threads (if that's what's happening) it might be sending a signal to Google that users aren't finding what they want on the page. There have been analogous discussions about Google Instant's Web Previews.
The thread titles apparently aren't well moderated or cleaned up... and semantically they are not helping Google classify the page. I see similar problems in much of the user-generated content sites that have taken a hit.
It's not that simple. Instead of merely thinking bounce rate, think about something more complex, like what other data Google has about the searcher and the query, and what the searcher does after the bounce.