(you should, of course, read read the judgement before reading any comments on it.)
The important paragraph in this result is:
One who distributes a device with the object of promoting its use to infringe copyright, as shown by clear expression or other affirmative steps taken to foster infringement, going beyond mere distribution with knowledge of third-party action, is liable for the resulting acts of infringement by third parties using the device, regardless of the device's lawful uses.
This is pretty vague, legal wise. Consider the design of Freenet. Node operators were unable to see what data was stored on their node. It could have been fragments of any file and we considered that a defence against "questionable content" (e.g pro-democracy docs in China). Now, imagine that Freenet ever worked well enough to allow for large scale file sharing. Does that aspect of the design open us up to an MGM lawsuit? It's an `affirmative step' taken to make the network difficult to police. Therefore the argument comes down to `we weren't thinking of file sharing when we designed it - honest!'. It seems pretty impossible to believe that technically competent people wouldn't consider that any communication system could be used for file sharing.
This ruling requires a lot of clarification before there can be any kind of checklist of what is illegal. In the mean time you have to consider if you want to develop any kind of network because you might get sued for it. That's exactly the permission world that MGM et al want. Change is a bitch for those who profit by the status quo.