I think especially in a forum where people tend to be semi anon. This isn't staking out a facebook name and keeping up with highschool friends. I know a few user names by sight on reddit but I really don't care if I hear from them again and I don't expect they care about me.
Makes leaving to anywhere that can put together a decent UGC interface pretty simple. It just feels like other than content, which reddit doesn't actually post, there's not much in the way of network affect.
For the longest time Reddit was predominantly the latter group of power users but in recent years Reddit has had a mass influx of the former casual group of users. I think the bet that Reddit is making is that the latter group is much smaller than the former and cutting off API access won't make a significant difference. But if they are wrong, this could well be the end of Reddit, going just like Digg went. It will likely still exist, but as a shell of it's former self.
I can’t imagine this killing reddit, not at all, but I think the power users and moderators are important to what makes reddit reddit and that without them the quality will decline quite a lot.