Although the Top Sites feature is dynamically generated based on sites you frequent -- and you'll likely visit those sites even more, since they're displayed every time you start up Safari 4 -- you can customize the page by "pinning" sites in place to keep them from being displaced. You can also exclude a site from ever being displayed, no matter how often you visit it.
The biggest advantage to this feature is that if you regularly visit the same sites for updates, you can easily keep tabs on them first thing in the morning or throughout the day without having to bookmark them. And if you're surfing along and want to get back to your Top Sites page, there's a new checkerboard-like icon in the browser bar for one-click access.
The visual effect makes Top Sites cool, but the functionality is what makes it useful.
A second feature that you'll notice in Safari 4 is that tabs are now displayed as part of the title bar at the top of the browser window. This is the feature that seems most heavily influenced by Google Chrome, though it's offered with an Apple flair. The design takes tabbed browsing in a direction it has been going in other Web browsers by making tabs more useful and accessible. At the same time, the minimalist approach reduces the amount of screen space required for tabbed browsing, a welcome touch on any machine with a smaller display.
One feature that I like immensely is that Apple included a visible, but inconspicuous, button at the right side of the title bar -- it looks like a small plus sign -- for creating new tabs. Doing so before required a key combination. Although this may seem like a minor change, it's likely to get users who don't know about the key combo -- CMD-T -- or who may never have tried tabs to give them a spin.
New tabs display with corners that can be used to rearrange them or pull them off into separate windows. It's another tweak that takes an existing option and makes it more accessible to new and experienced users. In both approaches, it's clear that Apple tried to provide new interface options that let users know features exist but don't to make them too complex. It's a smart balance between UI and usefulness that works. If you're not a fan of this new approach to tabs, there are ways of reverting to earlier interface standards. (Whether these will remain in the final release isn't clear).