I'm sure some guests don't realize that it would be nice if they put a name in... and some members have lost their cookies and didn't realize they weren't signed in.

A simple javascript form validation would check the guest name box, pop up a notice if it's not filled out, and only allow the post to go through if there's something in the box.

Then we'd have guests who just post "1" or guests who think to themselves "Oh, duh, I should add a name so that people can recognize my posts" and if that guest wants to be anonymous, well, he can just put in "anonymous". It's done on other forums and it works. Sure, "GUEST, anonymous" might not always be the same person, but a lot of people would choose a handle and stick with it. Then they won't get scared away by guest abuse from current members, and might actually register.

There's absolutely no need to require registration unless you want to.

Easy.