Take a look at the search field to your left. (You may have to scroll down.) With the "Recent Comments" button, you can get a listing of all the entries that people have commented on recently, so you can easily keep track of discussions on the site.
The search page has more options than the field on the left. You can do case-sensitive search and search for regular expressions. You can even search the comments now! Click the "Advanced Search" button to see the options.
(Finally, I sometimes attach keywords to posts, so you might get results even when the word doesn't appear in the text.)