A plug in / extension / script to allow users to filter certain topics would be brilliant, I think.
Allowing mods to limit the number of topics would just cause a great deal of meta outrage. It's bad enough when people flag. It'd be awful if mods controlled the front page.
Some sort of topic filter, or a tagging system might help (or use hnapp.com or whatever third party thing is available), but putting a hard limit on something that other people want to talk about is pointless censorship.
It's a discussion forum, and this seems to resonate with the community.
I found it in a comment from a few days ago ( https://news.ycombinator.com/item?id=5846416 ). It's a bit outdated, you should add "Snowden" to the filtered words.
(hint: To read the HN comments, click on the points/comments numbers.)
I think this is more to do with a persons engagement on the topic in question. Hacker News works by following the consensus of what the majority want to see. So I think it is working correctly.
1. That to place a limit on the number of articles on a particular topic on the homepage you must be able to group articles into topics.
2. If you can group articles into topics why not implement either: (a). a filter control which filters out topics based on user selection or (b). a second "diversity of topics" page HN which includes only the top articles on HN per topic (a log plot of HN).