I think you have to be a bit less simplistic in your approach.
Even those people who choose not to share may be doing that because they're nervous about using p2p...how many people do you think are just plain scared of being prosecuted? Are you going to tell them that they have nothing to worry about? Out of all those people, there are those who will start to share files...once they're familiar with whatever cilent they're using & feel a bit more comfortable with the whole concept of p2p.
If they were told from day one that they have to share, or they're not allowed to use the client...they'd run a mile & I wouldn't blame them. That would be the network's loss.
There are always going to be freeloaders...fact of life. But to be so militant about forcing people to share would only harm the client & p2p in general.

just food for thought.....

as mentioned above, theres always the issue of the ones which are powerless to reconfigure firewalls etc.... but never the less... have a read and see what you think.

im sure many of you have used azerus ( spelt right? ) and demonoid.com to download torrents... well demonoid have a little system for ratio which changes on how much you download and share... the more you download without sharing, the lower your ratio depends.. surely using a script to limit their download speed according to their ratio would be doable?

you could store ratio info on a remote server or on the client pc,

// for C++ programmers

// a file is loaded which store the ratio when GUI loads
TStringList *ratiofile = new TStringList();
ratiofile->LoadFromFile(encryptedfile.data);
Edit1->Text=ratiofile->Strings[0];


//turn the value in to an Integer
int Dratio =ratiofile->Strings[1];

//file download complete or done by mb
if (*file complete or mb d/l*)
{
Dratio++
}

//file upload complete or done by mb
if (*file complete or mb d/l*)
{
Dratio--
}

// then dependant of value GT or LT set speed
if (Dratio < 10)
{
*allow full bandwidth
}

if (Dratio > 10)
{
* slightly limit bandwidth
}

if (Dratio > 20)
{
*limit bandwidth more
}

etc etc

//then all whats left is to save ratio when closed or periodically
ofstream saveratio(encryptedfile.data);
saveratio<<Dratio<<endl;
saveratio.close();