Content "filtering" raises many privacy concerns. In its most egregious form, content filtering can lead to a "Big Brother" George Orwell "1984" scenario. For instance, in Cuba, if a computer user types a "dissent" keyword, the word processor or browser is automatically closed, and a "state security" warning is given1. Fear of this type of heavy-handed usage is what fuels the most vocal and enraged critics2 of content filtering.
ISPs in some countries like China3, Burma4 and Iran5 are required to block certain internet addresses as an extension of State policy or dogma in order to squelch dissent and and "control" citizens. (In all, some two dozen countries are doing "political" web filtering (as well as filtering for other social and societal reasons6.)
On the other hand, there are some legitimate reasons to engage in content filtering as set forth in the "Content Filtering" page here.
