1. Those who were net denizens pre-Outlook, who grew up with sophisticated mail and news clients that properly threaded discussions, along with strong cultural norms to reply inline, trim quoted messages, and keep signatures short.
2. Those whose first experiences with email were post-Outlook, for whom the norm has always been top-posting.
It's hardly surprising that each group regards the other with suspicion, but I think it's really telling that all the people who regard email as a bad tool for managing lengthy, in-depth discussions between multiple people seem to fall into the second camp.