I'll explain how it works on our forum.
First of all, its not a "last replays" feed, but "last updated threads" feed, so 1 reply (or 10 replyes in one thread) fill only one line of content. I mean 1 row of the table, becouse its a table actualy. Each row contains couple of cells - name of the forum (forum section), name of the thread, last poster name and last post time, so its quite compact. It has about 10 or 15 rows on main forum page, but also there is "view all answers on separete page" link, and that page, actualy, quite more usefull. It has auto-updating system, so people use it like a news feed, spending all day on it. In fact, there is a couple of people who still prefer "Show unread posts since last visit", but 99% of people use this new page.
Also it is useful on our forum becouse sometime there is a dozens new replyes in some specific threads, and not all people interestetd in reading them.
Here is a screenshot of that separate page, and main page has identical table but limited by 10-15 rows without all that additional functionality. Ofcourse i cant repeat this feature identicaly on SMF becouse it was a great job done there, but base functionality is quite simple. And we can allways make it srollable or something on the main page.