I can't speak to Second Life, but in World of Warcraft, once a player get a high level character, there's no way to progress in the game without the help of others. And to get to that point, a player has to spend significant effort (or money if they buy gold). Other players won't play with a bad seed, and so the cost is a stuck character and a large sunk effort.

What kind of shape do you think the "property" on the web/net will need to be to make it costly?

One problem I see is that reputation online won't ever really be much better than reputation offline. And offline, people don't really care much about their reputation. Not for the equivalent things online. I'm probably not being very clear with that...

Online, my bad behavior is limited (for the most part) to a couple of broad areas: being an ass, and being a thief.

The former is being rude, saying nasty things about others in blogs, forums, etc.. While it's much more visible online, the penalties online are similar to the penalties offline. One becomes known as an ass, and people tune that person out. In the real world, the ass moves on to a new group of people. Online, an offender gets a new identity sometimes, and more likely just moves on to a new forum. Penalties are similar.

The second is most often found in online auctions, and mostly in the nature of small time fraud. Similar penalties exist as with being an ass. But people act similarly in real life. Small businesses often survive with mediocre reputations. Small sellers online can as well. And ditching a name and starting over is fairly frequent in real life as online. Bad buyers, both online and offline, rarely worry about getting a bad name.

Most bad behavior penalized by property loss in the real world has no parallel in the online world. A bar fight? No analog. Poor driving resulting in a wreck? No analog. For most other kinds of bad behavior online, the only kinds of analogs in the real world that I can think of do not have penalties in property, but instead in freedom. Child porn online? Denial of service or other hacking? Spamming? These things either have an analog in real life that is punished by lack of freedom, or they have no analog at all. And they are all (that I can think of) characterized by anonymous perpetrators, much like burglary in real life.

In order to have a cost associated with bad reputation, you'd need to attach the property that could be lost to the identity and reputation. You'd have to get rid of anonymity on the net and/or reduce the identities one could have (a nearly impossible task given the net's infrastructure and culture) , and you'd also have to remove the ability to move on to another forum. Even if windley.com banned me from posting and that got noted in my global reputation, other autonomous net locations will freely allow me to talk there with a mostly fresh local reputation. At least if the ability to participate somewhere is the only property that can be tied to the reputation.

Increasing the property attached to an identity/reputation also has the side effect of reducing the participation online. Say goodbye to some of the online world's vibrancy and possibility. (Though that might be a worthwhile tradeoff.)

Only people with very valuable reputations will ever care about losing their reputations. Very similar to offline.

(Yes, kind of a rambly and not the most coherent of presentations of my thoughts. Short version is I'm not sure there really is a solution based on property in the online world.)