Would be great if we could use a custom field to set a different reply-to address for campaigns. There's a couple of specific advantages that would accrue to some of our clients, but I'm sure others might find other uses too:
1. It'd allow a degree of A/B testing with respect to email replies (or at least a better way to track them)
2. Where different segments are best dealt with by different teams/individuals it would mean we could channel replies directly.
I'd be interested to hear the ramifications with respect to false-positives in junk mail/spam detectors and whatever other issues might arise out of differing reply-to addresses, but if there's no major stumbling block here this would be a great feature!