I've just had an idea which may be awful, or may be helpful. I'm quite prepared to be downvoted to smithereens.
Should the home page actually be representative of the site at all? Who sees the home page, anyway? Do any regulars really go there rather than the questions page?
Why not have a home page which is almost entirely different to the questions page? In particular, there's nothing on the home page which says, "This is what the site is for. Here's why you might want to use it. Here's how you can use it." Something closer to the "about" page in some respects - but more focused on the newcomer than on the SO team and moderators.
There should be "quick exits" to the questions page, user registration etc - and it certainly wouldn't hurt to have a short sample of recent questions there... possibly even with freehand circles overlaid to show what each of the bits is about.
In short: consider changing the home page to simply be an entry point for new users, rather than something which is meant to be immediately useful to existing users. (If you wanted to do make sure the home page is useful for existing users, you could always display the questions page if you detect that the user is logged in...)
I completely agree that the UI for phpBB is awful. But being "better than awful" isn't a high enough bar to set yourself :)
EDIT: Yes, I know there's the top bar for first time visitors - but the rest of the page is still very busy (with information which is entirely useful for non-first time visitors, yes) and you have to click to get at the basic premise of the site. When I go to the home page of a new site, I don't want to have to navigate to a FAQ to get the general idea.
If regular users do genuinely still use the home page (and I'm obviously not the only one who goes straight to the Questions page - see Software Monkey's comment) and want it to show questions rather than a general intro, then I seriously suggest that the home page shows two radically different views based on whether the user is known or not. Don't just put a title bar up - change the entire contents. Yes, show some questions to give a sampling of what's on offer - but within a context which is more useful for a first time user.
I would suggest that if you were designing the ideal landing page for a first time user, the current home page is not what you'd come up with. So why not design that ideal page?