I think a big part of the difference between 'old fashioned' porn - even video porn, but especially skin mags etc, comes down to imagination.
Erotic media has always been available. However, until recently, it was an aid to masturbation. Viewing was supposed to titillate. To inspire. A person would see these images and bounce off them, create their own psychological erotic landscape.
Now, we are dealing with the idea that any fantasy, any specific desire, any combination or look, can be found, in HD detail, online, at any moment. That is what we're encouraged to do - to define our interests by taking them outside ourselves, to a place where we're passive observers, rather than actively invested.
For a lot of people, this might not matter. Reality remains more appealing. But I think for a significant subset - maybe people inclined this way already, maybe people who for whatever reason find themselves watching more porn than others, maybe other reasons - this availability, this lack of the need for personal input, becomes part of their sexual and emotional makeup. And I think this can make things difficult, when it comes to 'real' sexual life - where things tend not to go so much to plan!
fulb, I totally understand why you feel the way you do. A lot of people I talk to find the concept of porn being damaging very hard to grasp, perhaps because either it doesn't damage them, or they don't think it does, or for whatever reason. My opinion, though, we are seeing more people and relationships damaged by it, and I don't think it's something people want to admit to or discuss.
I honestly urge you to check out the reddit board - it's a really good example of the types of regular, smart, decent guys who've found themselves unable to control their porn watching.