The new website is now live for everyone. Please let me know if you see any issues.
I never responded to the (massive) thread on preventing Reply to Group/Sender mixups, but I did read everything and have made some changes based on your feedback, which you will see on the site now.
If the group allows it, there's now a toggle button at the bottom right of the reply box. Clicking that button toggles the main reply button on the left. This is similar to how several other services handle this issue, and I think is an improvement over what we had before.
Also, for replies to sender, the word 'Private: ' is prepended to the subject line.
I played around with having some way of selecting where your reply goes to before the reply box opens up, but was unhappy with everything I tried.