gtrot.com started with Facebook-Connect login only. We didn't allow any access into the site unless you created account. The primary reason for this was that we used Facebook data to show you where your friends lived, where they've been and allowed you to connect with them on gtrot (through Facebook) so there was no "empty room" feeling when you started.
With our latest updates in November, we allowed a 'generic view' --- a user can search what's happening in a city without logging in. We've put in educational messaging and explain why the Facebook connect is important to being able to personalize results. Based on what you like, what your friends like, where you've been, where they've been etc. The data is core to the experience, but some people still don't want to give access to Facebook. We did, however, see our bounce rate cut in half when we provided a generic view. Customers were spending more time on the site (upward of 5 minutes) and even returning to use the site without the account.
I think the key is giving clarity, but this is a topic we've gone back and forth on as we don't think we can deliver on our promise of 'personalized advice in any city' without having the Facebook basis. We'll be changing this over time but for now, it's generic or Facebook account.