There are many, many circumstances in which computer-matched ads might not be a good fit. Imagine if a site for entomologists somehow got stuck with ads for Volkswagen Beetle parts, for example. Or if a site meant to showcase funny pratfalls was paired with ads for vacations ("trips"). Fortunately, Google's testing expanded category blocking with AdSense.
The category filtering beta used to offer sites' owners a way to turn down ads from up to five categories. It was meant to be used in a rather broad fashion.
Now, a post on the Inside AdSense blog has explained, "General category blocking will allow you to block even more categories of ads from appearing on your site. We're testing this feature with 170 more specific categories, including finance, travel, job, and automotive. This feature will allow publishers to block up to 50 ad categories from among a broad range of 170 fine grained categories."
Also, as before, the feature will show publishers what sort of impact changes would have on their earnings.
Note that not everyone will get to take advantage of the new options; they're only available through the new AdSense interface, and for that matter, will only work in combination with English-language ads.
Still, the feature shows a lot of promise, and Google plans to polish it and increase its availability in time.