As I know a lot of you developers agree, Reply-To: header munging is not very nice. However, it appears to be a necessary evil sometimes.
My biggest problem with Reply-To: header munging is that there is no way for a poster to direct replies off-list. Any attempt to do so will fail completely because MUAs obey the Reply-To: header, and the users don't notice. Ironically, these users are often the ones who made the munging necessary in the first place.
However, I believe there is a solution to this problem. How about only adding a Reply-To: header only if one did not previously exist? This way, headers will be munged in the normal case, but if someone adds their own Reply-To: header for some reason, their value will be kept. Naturally, this behaviour should be optional, since it alters the behaviour of the lists.
I have attached a patch which enables this functionality. The patch is against 2.1.9, but seems to apply on 2.1.10b1 without complaining.
PS: This patch slightly changes the order in which munging is done, causing mlist.reply_to_address to be appended instead of prepended to the original Reply-To: header. I didn't consider this a problem, as it makes the behaviour if "This list" and "Explicit address" consistent. I also feel it made the code more straight-forward to read.