The biggest new feature in Snow Leopard is support for Exchange, Microsoft's popular e-mail, contact, and calendar server. The iPhone got support for Exchange last year, and now it's the Mac's turn to join the party. (Ironically, Windows doesn't support Exchange out of the box.) As a result, it has become that much easier to integrate the Mac into businesses that have standardized on Exchange.
The way Exchange support works in Snow Leopard is pretty simple: once you add an Exchange account in Mail, you'll start receiving e-mail messages in Mail, you'll be able to view Exchange contacts in Address Book, and Exchange calendars and tasks will appear in iCal. iCal will even perform tricky tasks like scheduling meetings based on the free/busy status of invitees; you can accept or decline meeting invitations right from Mail. If you're syncing your iPhone to the same Exchange server, all those calendar events will sync up automatically.
Apple's not trumpeting the feature, but Snow Leopard does actually include a certain degree of built-in protection against dangerous software. The same system that Leopard employed to warn you before you open programs or mount disk images downloaded from the Internet now also checks those files for known dangers.
It's a testament to the limited number of Mac OS X malware threats that Apple's stock list of dangerous files contains all of two entries. However, that list can be automatically updated via Software Update, so it definitely provides a first line of defense against unwittingly infecting your computer with evil software. However, once you're infected, Snow Leopard doesn't have a system for removing that malware. As a result, we expect that there will still be a strong market for third-party virus-checking and -removal software.