Certainly if content is not illegal, there is no obligation to take it down, whether we like it or not. See "No One Is Obligated To Take Down Perfectly Legal Content You Don't Like" (3 September 2008).
However, that is not really our point. Blocking access or asking content to be removed should go before the courts or at least an impartial body/institution/authority who could hear both sides and decide if the suspicious or litigious content is really illegal. That would save us from unregulated censorship (censorship for printed publications used to be regulated - even if nobody really agreed with the principle of censorship). After all, for movies, whether it's porn or not is the decision for most countries of accreditated bodies; why should it be different on the internet? A question of unpracticability (too much requests)? no study has been made and if nobody tries, we're just then giving up on fair trial's rights. See Thailand and China "Thailand Continues To Try To Mimic China With Internet Censorship" (4 September 2008)
in that sense, the US DMCA takedown notice is more respectful of rights than simple requests: it must meet some important conditions that, if met, compel the ISPs or host to takedown the litigious copyright material. See "But What If A Takedown Notice Isn't Actually A DMCA Takedown?" (22 August 2008) and "Judge Says Copyright Holders Must Consider Fair Use Before Sending DMCA Takedowns" (21 August 2008)