I'm so happy. While SBNation has a great blogging platform, I've always been annoyed at how threads immediately die once they leave the front page. I just found the answer though: the archives.
By clicking on the archives, you'll be taken to a looong list of front page posts sorted with the most recent posts at the top. Best of all, every post with new comments will tell you how many you haven't read (if you're a member of BtB). Something like this:
One downside, however, is that FanPosts and FanShots won't appear in the list. To find older FanPosts, you can scroll through the Recently Active tab on the FanPosts page (which is the default view). To find older FanShots, you'll need to scroll back through older pages on the FanShots page. If you increase the number of FanShots shown in the sidebar to more than 10, that number will also be the number shown in full on each historical FanShots page. You can also change the number of Front Page posts and FanPosts shown on the front page beyond 10. I don't think any of these methods will show you historical FanShots created by BtB authors for the Front Page, though.
Now you have no excuse not to keep up with the comments in old threads. They're all just one click away!