Very helpful post. Thanks.
First, to correct a slight misstatement in my previous post...
Linking to too many posts will drain the link juice available for the deeper posts.
I should have said, "linking to too many threads".
Your description of your home page suggests to me that directly "linking to too many threads" from home, combined with your rapid turnover and too much of the text content on home, is likely a good part of the problem....
It features the 10 latest new topics in full. These change frequently, so each one would be on the homepage for no more than a couple of hours.
There are 50 links (in total) to latest topics (titles only) in the main categories.
Rushed version of my answer for now...
- way too many deep links from home to individual threads (60 in all) in comparison to only about 30 to your main navigation system (which should be feeding all the threads in all the rest of your site).
- much too much post content on home. Inclusion of those topics "in full" encourages Google, as you've noted, to rank your home page. Also, having the complete topics on home also doesn't encourage visitors to use the deep link, as a snippet would... so you may not be getting all the deep inbounds you might have if you'd showed a snippet only.
- Related to this, frequent change of latest topics may only confuse Google. I would not change them nearly as frequently as you're doing, but this depends on your area. In some cases, that much freshness may be warranted, but I don't know your area.
Note, though, that Google doesn't somehow save the previous position of your link. When you change the link, you've changed the entire link juice situation for Google.
That very brief exposure on home also may not be helping you build link equity with visitors. A lot depends on your traffic patterns.
I'd keep the overall count of direct deep links to threads to maybe 1/4 of your overall home page linking, and post titles and a small snippet for each. I prefer a snippet uniquely written for the purpose as opposed to the first line of the post.
Hottest threads and threads you want to promote for their potential are the ones that should be on home, long enough for your visitors to interact with them. Don't automatically put newest threads on home. Perhaps make greater use of category pages. Again, though, you may be more news oriented than I'm thinking. Usage patterns should influence structure. The above should give you a sense of how I think they might interrelate.
I'd approach fixing this gradually, testing each step of the way. In the transition you want to minimize losing traffic to home before you build traffic to inner pages. Perhaps gradually reduce post content on home and number of direct links to individual threads, and beef up your category pages as you do this.