Ah! I didn't see it because I was already logged in.
Two problems here. First, the people who are already registered
generally know how to get around the system. The people who
aren't registered, however, will have no idea that they can
register by clicking on "Login." We need an explicit link for first
time registrants.
The second problem is that when I tried posting as a
non-registered person, I got a "not authorized" error message
with a small link asking me to log in "as another user." If a
non-registered person (or even a registered person without
much savvy) saw this message, s/he would never figure out
what to do in order to register and complete the posting
operation.
