ISPs have to proactively police copyright on user-contributed material. This means that it will be impossible to run a service like Flickr or YouTube or Blogger, since hiring enough lawyers to ensure that the mountain of material uploaded every second isn't infringing will exceed any hope of profitability
Sites like YouTube have enough clout that I don't expect them to simply be shut down outright. On the contrary, their backing companies will be invited to the negotiating table, where they will help craft a treaty that has enough exceptions for them to operate.
That's great for YouTube, but there is a chilling problem for future innovation. It is unlikely that the next great network service will comply with whatever exceptions are carved out. Thus, if ACTA goes through, large swaths of potential society-changing services will end up requiring an act of congress to even get off the ground.