So maybe its just me, but if you're trying to run an open source project, something that basically lives and dies by random people showing up on its mailing list and making contributions, you'd actually want to go out of your way to make sure it's easy.
When I try and post a mail to your mailing list, and I get a reply back saying I have to subscribe to post, that's a huge turn off. I mean hell, in this case, I was actually subscribed, just under my main email address, not my gmail address. Luckily, I happened to know the mailing list software this list was using, so I knew how to work around it, if I hadn't then I probably would have given up after about 30 seconds and gone and done something else.
Any time someone trying to contribute gives up and goes and does something else, you lose. You lose that contribution, and you lose out on any future contributions that person might have made. Sure, sometimes that person's an idiot, and would be more trouble than they're worth, but sometimes it's the next huge contributor whos participation turns into a big win for the project, but you never know if they give up after they fail to send that first mail.
I mean if you can't get someone to volunteer to moderate your list, maybe you should reconsider the need for the mailing list in the first place...