The biggest of these is the new "Top Sites" page; a panorama of "previews" of the sites you visit most frequently. This feels a lot like the "Most Visited" page in Google Chrome, but it's flashier and has more functionality. For one thing, you can "Pin" Top Sites so they'll always be there. Also, if one of your Top Sites has been updated since your last visit, the top right corner of its preview will be folded down to reveal a star. This lets you quickly scan favorite sites and identify new content. Clicking a preview takes you to the site, and the bookmarks toolbar has a new "Show Top Sites" icon to take you back. (Tip: You can also access Top Site via the "url" topsites:// — you can use this to set a Top Sites bookmark or set your homepage to the Top Sites page.)
The second big change is the location of tabs for tabbed browsing. The tabs now appear at the very top of the browser window (again, similar to Google Chrome). The 'close tab' icon is on the far left, and a 'handle' icon is on the far right of each tab. By grabbing this handle you can rearrange tabs or tear them off into a separate window. Conversely you can take the single tab of a freestanding window and drag it into the tabs of another window to merge the two. You could do all this with Safari 3 but the operation of merging a single window with a tabbed window was clunkier. You actually had to open a 2nd tab in the single window to get the tab interface to show. On the other hand in Safari 3 you could grab any part of a tab to manipulate it, while in 4 you have to zero in on the handle. Not a big deal but it takes some getting used to.
Another downside of the new tabs is that a page's title appears only in the tab. If you have a lot of tabs open you won't be able to read the entire title unless you hover your mouse cursor over the tab for a few seconds (this is also a limitation in Chrome).
Viewing your bookmarks or history has gotten an update as well. This information is now displayed using the "Cover Flow" system that we first saw in iTunes and now is used in the OS X Finder for all kinds of files. The jury is still out on how much value this adds; to me, flipping through your bookmarks feels slower then scanning a textual list.
Under Windows you can hide the menu bar to give Safari a very small UI footprint. Again, (Dare I say it?) similar to Google's Chrome. But where this is an option in Safari, in Chrome there is no way to turn on a menu bar if you prefer that ease of access.
The Mac version of Safari 4 Beta requires Mac OS 10.5.6 with all current security updates. Keep in mind that installing the beta will clobber Safari 3. The Mac installation package (both Windows and Mac versions can be downloaded from the Apple website) includes an uninstaller application that will remove Safari 4 and revert you back to version 3 (I tested this and it worked as advertised). The Windows installation seems to be less fussy, though on one XP machine the Top Site functionality wasn't there. I don't mean it didn't work; the option simply didn't seem to exist (on a second XP machine all was well). The problem XP installation was running under VMWare Fusion on a MacBook Pro, so that may have had something to do with it; we couldn't duplicate this "missing feature" phenomenon.
I'm traditionally a Firefox person, but after running Safari 4 for most of the day today, I'm going to stick with it for a while. It feels very snappy, and it has handled every site I've thrown at it so far (and although it's a beta, I haven't had any crashes yet). I can't totally discard Firefox though; I rely on too many extensions for my day-to-day work. I'd also like to see more interface options in Safari. For instance in Firefox there is a "New pages should be opened in a new tab" setting and I can't find a similar option in Safari, and so I have to use some kind of modifier key in order to keep all my web pages in one neatly tabbed window. I can adjust (or just use a mouse instead of a trackpad) but it's curious the kinds of little things that end up meaning a lot.
Bottom line, Safari 4 Public Beta is well worth a look; there are a lot more new smaller features and improvements beyond the few I've covered here. Whether or not you end up sticking with it will probably depend a lot on whether you can get by without your favorite Firefox extensions. Both Safari and Chrome need to tackle this problem. If you don't use Extensions, then the choice between Safari and Chrome (under Windows for now, but Chrome should be coming to OS X in the near future) will be an interesting one.
Top Sites is the biggest new feature of Safari 4 Beta