settings have changed from times to times. I'm quite pleased that the current mailing list setting is that anybody is allowed to post to clisp-list without requiring registration & membership first. This has not always been the case in the past, and I don't know for how long the current settings have been in effect.
The current setting makes it easy for casual users to ask questions and report problems. Sourceforge's spam filter seems good enough for now at eliminating almost all unwanted posts.
A small drawback is that I recommend (and AFAIK general netiquette considers it polite to do so) that replies be CC:'d to the original poster, for s/he may not be a member of the mailing list. Of course, you don't need to do so when replying to regular contributors (i.e. known members) of this mailing list ;)
I'm very much in favour of open mailing lists, as we had >10 years ago before spam abuse caused a change in most policies. In effect, spam is a successful DoS on worldwide e-mail service: open lists became member-only, causing a loss in communications among people. For instance, there's a bug I'd like to submit to the sbcl mailing list. But I need to register first and haven't yet done so.