As someone whose job involves making sure a number of Macs play nice on a Windows network, I'm very curious to see what kinds of things Apple has done in that regard in Leopard, although I'm concerned because they haven't commented on that much. In the past, they've made a big deal out of improvements to interoperability with Windows networks and Windows machines but this time around, it's been a lot quieter. I hope that doesn't mean Apple has neglected that end of things.
I'm particularly interested in two areas: how much better Address Book interfaces with Exchange servers and LDAP servers, how much better Mail integrates with Exchange and whatever improvements may have been made to Directory Access, in particular the Active Directory plug-in.
Those aren't very sexy topics, but it's stuff like that that keeps Macs viable in Windows network settings (which is the norm for most businesses) and hopefully Apple isn't neglecting that stuff this time around.