More questions than answers
Personally I have very little interest in what the IWF thinks might be illegal, because it is not a body competent to judge such things under UK law. That said, I am sure there is a benefit to them making their best estimate and making available a list of URLs which it thinks might house potentially illegal content. That way, those who are concerned can consult such a list and take action accordingly. Some might even choose to empower a third party to monitor such a list and take pre-emptive action on their behalf. I wouldn't, but I can imagine that some might.
It is a huge leap from that to a situation in which an ISP takes this non-legal opinion, and implements a transparent proxy of the "offending" site which alters their users' interaction with that site making pages appear missing, AND FAILS TO INFORM THEIR USERS THEY ARE DOING SO.
It is a direct analogue of the Phorm situation - users pay for connection to the internet, not a connection to a version of the internet amended for the benefit of a third party. Users have every right to be angry and feel let down about this. ISPs simply should not be transforming content.
As for the other mealy mouthed points in this opinion piece, they are dubious at best. The Wikipedia page in question housed a thoughtful and rounded discussion of the album art, putting it into context and exploring the criticism it had received. It is simply not good enough to say that this textual content was censored for simplicity's sake. It is no harder to add a .jpg URL to a watchlist as it is to add a .html URL.
Witness the difference between Demon Internet's action, which was the show the page in its entirety and then simply *warn* users that clicking through to the full size image might be illegal, and Virgin Media's, which was to transparently proxy the page in question and pretend it simply didn't exist.
The bottom line is this: I didn't sign up for a censorship service. I don't want a censorship service. If I want to read stuff that someone else thinks I shouldn't it's down to me to decide. And if I want to view an image that someone else thinks I shouldn't, even if they think I'd be committing an offence if I did, it is still MY DECISION to commit that offence if I want to.
I don't care how righteous the IWF thinks it's being, nor how indignant Wikipedia is. I care that my internet connection delivers me what is actually on the internet, not some nannied version of it.
That's why there was and continues to be an outcry over this.