Users sharing is very important, but in this day of lawsuits and RIAA-induced paranoia, one of the requirements future p2p file sharing protocols will need if they want to be succesful is anonymity. Today, the chance of getting caught and sued is pretty slim, considering the humongous amount of users the larger p2p networks have. And if you're outside the United states, it's even smaller. Still, it's going to increase, and it's enough to scare off many potential users. I've heard it plenty of times: "I don't download music because I don't wanna get sued".

Take a look at KaZaA: it's the most used network (although regarded by many, myself included, as a noob's p2p). However, it provides no anonimity whatsoever. Everything's done in plain air, and anybody can swipe an IP number and either ask or threaten the ISP for the user's personal data, or sue à la "John Doe", as the RIAA is starting to do now that many ISPs have taken a stand against them. More obscure networks such as OpenNap are still in plain air, but few worry about them as they're not known enough to attract the kind of attention KaZaA does.

The Direct Connect network provides an additional obstacle for ill-intentioned lawyers/organizations, as you have to get through the hubs to hit the users. This means more lawsuits, and most often the hubs all have the usual "you take full responsibility for what you share" disclaimer, which murkies the waters even more.

However, things are starting to move. A recently born p2p called EarthStation 5 provides full anonimity. No public IPs, no way to identify specific users. I don't know whether it'll rise to fame or be forgotten by everybody, but it is a step in the right direction, soon to be followed by many others (hopefully).

My opinion is that this is the future of P2P. Of course, when it becomes a reality, we'll have a dire battle between filesharing networks to keep their users' anonimity and the RIAA (and associates) to crack their security. However, I believe they don't stand a chance, as they've repeatedly shown themselves to be hopelessly lagging behind as far as anti-p2p techniques go (so you get a corrupt file or three when you use KaZaA, big deal). All of their annoucements about horrible malware that gets distributed through KaZaA and erases your illegal mp3s, viruses hidden in the music files (yeah, right), and threatening emails have accomplished pretty much nothing.

We'll wait and see...

Update: Interrobang tells me Freenet has come out with an interesting way to keep the users from being sued. You can still be punished for sharing and requesting files, but it's impossible to prove you did it. I'll add more information soon.

Well, a long time has passed, and I never added any information. I'd do it now, if there wasn't a node for that already.
Personally, I have tried Freenet, but I found it impossibly slow, complicated and unreliable. It was, admittedly, a rather half-arsed attempt; I probably missed something, but I didn't feel like going through pages of documentation to get stuff Google already finds for me. Should I ever develop an interest in highly illegal stuff, or should Italy start banning the Internet as South Korea is doing (highly unlikely, but you never know... we are living in a time when most governments would really love to erase any form of personal freedom), I'll probably be far more interested in the project.

Also, it seems my predictions haven't come true. ES5 no longer exists (its website has a pathetic 404 as its main page), and even when it did it was little more than a hoax, leeching p2p services from other networks.

Anonimity in P2P networks still doesn't seem to be on top of the agenda for most of those who are developing such programs. I think this is in part because it's really hard to anonimize file transfers without proxies or other intermediate action, and in part because the RIAA are moving on to other ways of fighting the phenomenon.
Incredibly, they seem to have realized suing single users wasn't doing them any good. They are now trying to "sensibilize" the people about the horrible, horrible crime that is downloading music or movies: I'm sure you've seen those ads after movie trailers in cinemas (to which I usually reply loudly "Nice ad, gonna download it when I get home", prompting laughter from fellow-minded individuals). They are also trying to get ISPs to limit/shut down ports normally used by P2P services. The result is that a single user is even less likely to be sued now.

Unsurprisingly, they are still losing the battle.

Anyway, I guess UberGeek was right all along: what counts most is that users understand the philosophy behind P2P and share as much as they can. Luckily, all serious clients (as in, those using the DC network and eMule/eDonkey) have ways of making it hard to give without taking. Really, when duping the system into letting you download requires you to use hacked clients, fake sharelists, custom configurations and so on, you might as well rip a few DVDs and share them. It'll take you less time and not being kicked/banned every ten minutes from DC hubs is likely to considerably reduce your frustration.

Please /msg me about any misinformation, errors, or anything wrong I might have said in this writeup and I'll correct them as soon as I can.