I've twice had users of the web interface complaining that there is no way to 'Reply to Sender'. I have to explain that it is there, but you need first to reply (which is not obvious, as it feels like reply to group) and then notice the greyed out button on the right "Private" and realise this is the same as reply to sender, and that it is not greyed out because not available but because the option is not selected.
It seems sufficiently obscure that people can't find it even when looking for it.
The email interface is fine, as it has a 'Reply to Sender' link as well as a 'Reply to Group' link.
I would prefer to have a button labelled "Reply to Sender" alongside "Reply to Group" and get rid of the Private button. I realise this gives something of an issue with the BCC me option - how valuable is this? could it be shown anyway and ignored for Reply to Group.
Or to have both Reply to Group and Reply to Sender options under the message display a level up.
Another option, less of a change, would be to alter the Private button to be a check box above the Reply to Group button, that way it would be more noticeable and more clearly associated with the Reply button.