Windows 7 inherits BitLocker drive encryption from Vista with one important update and one significant limitation. The limitation is that BitLocker, which can encrypt an entire disk or any chosen files or folders, is only available in the Ultimate and Enterprise editions of the operating system. The improvement is that, if you have Ultimate or Enterprise installed, you can now encrypt USB thumb drives as you would a hard disk. That's a nontrivial consideration, and one that could drive the selection of a Windows 7 version.
BitLocker can be controlled through AppLocker, a Windows 7 Enterprise feature that permits central control of the applications allowed to run on a system or group of systems. AppLocker is, along with Direct Access (a secure remote access method that can eliminate the need for third-party VPN software), a significant step up in corporate security for Windows 7.
Both Windows 7 and Snow Leopard have built-in firewalls, and each operating system allows for fine control over resource sharing, computer identity display, and network discovery. Many commenters have made the point that Windows 7 security is more advanced and capable than the security built into Snow Leopard. That is almost certainly true. It's equally true that the number of exploits attempted against Windows machines versus Macintosh computers make stronger security a more stringent requirement for the Windows system.
There's another thing I have to mention: system updates. In the time I've had Windows 7 installed, there has been a regular stream of patches and updates. I know that patching vulnerabilities is crucial, and I applaud Microsoft for moving quickly on this front, but the fact that I am regularly informed of updates plants questions in my head over security and reliability. Snow Leopard has been updated, yes, but at far less frequent intervals than Windows 7, a pattern that each operating system is continuing from its predecessor.
Verdict: I'd love to have a clear winner on this point, but in considering everything I'm going to call security a draw. Each system is better than its predecessor. Neither is perfect, but each will allow me to work without constant fear of data loss. It's a good draw, but a draw nonetheless.
Compatibility: Role reversalIn some ways, it's a bit early to draw conclusions about overall compatibility with existing applications, but a couple of statements can be made. First, I have been surprised by the number of Mac applications requiring new versions to work properly with Snow Leopard. For an operating system upgrade that did not carry a new user interface or entirely new code base, the number of bent and broken applications has been large. From Microsoft Office applications through Firefox and even less popular applications, many apps have required new versions in order to function with Snow Leopard.
So far, I have found very few applications that refuse to run with Windows 7. There are a couple, including the ZoneAlarm firewall, but most of the personal productivity apps have run quite well to this point. I suspect there will be a flood of new application versions that take advantage of the updated user interface presented by Windows 7, but those are upgrades, rather than patches to allow basic functionality.
Verdict: We're a long way from Vista. The compatibility points go to Windows 7.
Built-in apps and utilities Apple made significant improvements to Mail, Address Book, and iCal in Snow Leopard, bringing Exchange 2007 compatibility to these built-in apps. When you add the functions of iLife ($79), many of the basic communication and media production tasks most consumers (and many business users) will need to accomplish can be handled without spending a whole lot extra on application software.
Microsoft has taken the opposite approach, stripping Mail, Movie Maker, and other basic application software out of Windows 7. This succeeds in making the footprint smaller for users who don't want or need these functions. If you do need them, Microsoft has made Windows Live versions of Mail, Movie Maker, Writer (for blogging), and other applications available for free download. The new Live apps seem quite good, but obtaining them requires another step for users to go through.
Here's another small, but telling, issue with the basic applications from each company. While the Mac's Mail has been improved to work with Exchange Server 2007 out of the box, Windows Live Mail requires that the Exchange Server use the IMAP 4 protocol (rather than the native Exchange protocols) to delver mail. I'm not sure why this is so, but if you want to have Exchange functionality without buying additional software, then the Mac is the better answer. Go figure.