Here's the beginning of the stable I want for a forum:
1. Full (or nearly full) screen width post hierarchy display. The tree-like display you have now is actually pretty good, but obviously not full screen width. You can make up for the lack of room for ads by sticking them in individual posts when they're displayed (more on that in a moment).
2. Instead of showing the currently viewed post at the top of the page, show the post within the context of the hierarchy. When I click on a post title, I'd like to see the post appear in place. This allows for greater post display width, too. When the post appears, you can stick an ad on the right-hand side, using that extra width to good advantage to keep sponsors happy.
3. No reply depth limits. Show a vertical slice of the hierarchy that shifts to the right when viewing posts that are near the maximum width that'd fit in the screen, so that discussions can have greater depth metaphorically without a bunch of hassle trying to relate replies to the comments to which they respond -- without making the page "ugly".
4. Provide a separate print view that actually prints out cleanly without all the extra gunk on the page that wastes printer ink. Optimize the appearance of it for print media rather than screen media. This appearance optimization will have the added advantage of discouraging the use of the print view for viewing online to skip the ads.
5. Dim the background color slightly. Maybe #dddddd would be good. All this bright light coming out of the screen isn't optimal, and we don't even get the benefit of a tan with LCDs the way we did with CRTs.
6. Provide an alternate view, somehow, that doesn't require JavaScript. Preferable would be page design that degrades gracefully so that when JavaScript doesn't work, it just displays properly without any additional work on either your part or the user's.
7. In the JavaScript-enabled view at least, bring up the reply fields when needed attached to the bottom of the post to which we reply, rather than opening it in a new screen.
8. While you're at it, allow the user to specify more than one post to have open on the screen at the same time. This would allow for more effective and easier reference to other posts in the discussion when such is needed to construct a response.
There's another, simpler way to explain a good interface (though slightly different from what I described) -- just copy the discussion interface from [url=http://reddit.com][b]reddit[/b][/url]. That'd be keen enough, with the exception of a couple of enhancements I'd like to see:
1. Add that orange "NEW" tag to all comments that are up to three days old.
2. Differentiate read posts from unread posts with a text tag as well -- something not only clearly visible, but easily searchable with a form that is unlikely to be duplicated in the text or title of a comment the way "NEW" works.