Reply-To does not take care of anything nicely. There is no "default" behavior about how to reply; you have to think about whether to reply privately or publicly based on the topic and your intended content. (If anything, the default should probably be privately, unless the response really is of interest to the whole multitude of subscribers. All too often, mailing list discussions devolve to the point that it's not the case.)

Reply-To: stomps over the option of replying privately. It can still be done, with manual steps. Worse, someone might not be paying attention, and just use Reply out of habit, thinking it's a private reply, when in fact it is being broadcast to the list. It's very sneaky!

The old style is better because it is more convenient and non-broken. It keeps conversations intact by letting people have a debate with the mailing list without subscribing to it, and doesn't rudely re-program your Reply button into doing Reply All.