Just came across this piece on pay walls by Scott Rosenberg. His thesis? “It’s not the pay, it’s the wall”:

The problem is that the steps publishers take to maximize revenue end up minimizing the value and utility of their Web pages. Building a “pay wall” typically means that only a paying subscriber can access the page — that’s why it’s a wall. So others can’t link directly to it, and the article is unlikely to serve as the starting point for a wider conversation beyond the now-narrowed pool of subscribers.

This is an important thing to keep in mind when considering not just pay walls, but also registration walls, where readers must register to comment on or even see content. Be very careful that you don’t put so much of your site on the registration-required side of the wall that there’s nothing left for anyone to discover or share.