While it doesn't make a lot of sense to highlight every single change that's happened since the Beta, here are the changes that I think are most noteworthy. Some of these have been documented previously by Microsoft, while others are changes I've observed on my own.
User interface changes
Nothing, perhaps, is more controversial than the Windows 7 user interface. Obviously based heavily on the Windows Vista UI (which is yet another a reason to believe rumors of a coming UI/skin change), the Windows 7 UI has been tweaked, prodded, and squeezed in every way imaginable. The goal, of course, is to make it work they way you want it to work. We'll withhold final judgment on that one for the final release, but the UI as we see it in the RC is pretty and highly usable.
Desktop and Windows Explorer
ThePeek feature, which allows you to "peek" at open windows or the desktop while hiding all of the other open windows, has proven to be one of those wonderful stealth features that you never realized you wanted until you got it. In the Windows 7 RC, this feature has been added to taskbar button overflow lists (see below) and to Windows Flip, the ALT + TAB window switcher. So when you tap ALT + TAB, the Windows 7 RC utilizes Aero Peek to display only the selected window.
Windows Flip (ALT + TAB) now utilizes Aero Peek.
Microsoft has settled on the name Aero Themes and now the Windows 7 RC includes many more themes and sound schemes than were present in the Beta. The RC also includes locale-specific themes that weren't present in the Beta; these include new wallpapers, sounds, and Aero Glass colors that are locale-specific.
The Windows 7 RC includes a ton of high-quality Aero Themes.
While Windows 7 dispensed with the Vista-era taskbar early and on and put Windows Gadgets directly on the desktop, the RC introduces a few new behaviors. First, if you add Gadgets by double-clicking them in the Gadget Gallery, they will line up against the right side of the desktop, as if the Sidebar were still visible. This is handy if you miss the Sidebar, and of course you can always move them around as desired too.
Additionally, you can separately toggle the display of Gadgets and icons on the desktop via the right-click menu. So even if you choose to hide icons, you can still view Gadgets there.
(Note that no new Gadgets have appeared, or will appear, since the Beta, and that Windows includes just one Gadget--Windows Media Center--that wasn't present in Windows Vista.)
All Windows Explorer windows (including Libraries) will provide a parent folder button in the address bar so that you can go "up" in the folder hierarchy from the current location. Additionally, Windows 7 supports the ALT + UP ARROW keyboard shortcut to accomplish the same action. (Like ALT + LEFT ARROW for "go back" and ALT + RIGHT ARROW for "go forward.")
Speaking of Libraries, Microsoft has improved the unique header area that appears in Library windows to better emphasize the availability of (also unique) Arrange By options. These options, incidentally, provide for some pretty stunning views, and these views are not available to normal Explorer windows, just Libraries. Additionally, the Library header displays the name of the current sub-folder, if you've navigated into one.
Windows 7 Libraries have been enhanced and include unique Arrange By views that aren't available to normal folders.
Windows 7 does not allow applications to pin themselves to the taskbar during installation, and it hides the old Quick Launch toolbar that so many apps try to utilize as well. So Microsoft added a subtle change to the Start Menu in the RC so that newly installed applications appear for a short time at the bottom of the Start Menu MRU (most recently used) list (the left half of the Start Menu). This gives you the chance to discover newly installed apps and then pin them to the taskbar or Start Menu if you'd like.
After Microsoft Office has been installed, Excel pops up for a short while just to let you know it's there.
I complained long and hard that the default taskbar view style in Windows 7, in which a single icon with no accompanying textual label can often be used to represent multiple open windows, is a huge mistake and should be changed. Now that I've been using Windows 7 for several months, I feel even more strongly that this new default taskbar view is a user experience disaster. Unfortunately, Microsoft does not appear to agree: The Windows 7 release candidate ships with the same terrible default view style that was present in the Windows 7 Beta.
Tip: To fix this, right-click the taskbar and choose Properties. Then, choose "Combine when taskbar is full" next to Taskbar buttons. Then click OK to enjoy the taskbar the way it should be.
There have been some taskbar changes since the Beta, however. When the number of open windows triggers a taskbar thumbnail overflow, in which the live previews are replaced with a textual list of open windows as seen below, the list itself and the items it contains behave more like the live previews, with item "peek," and mouse-over list preview.
Taskbar button overflow lists now act more like normal Taskbar button previews.
On a related note, the taskbar has been enhanced to scale better when there are multiple buttons displayed. That is, the Windows 7 RC taskbar can contain more buttons than the Beta version. Microsoft says that on a 1024 x 768 display, the RC will display 25 percent more icons than the Beta version. If you switch to Small Icon view, the disparity is even greater, at 38 percent.
The Windows 7 taskbar also picks up the old Quick Launch keyboard shortcut series (WinKey + 1 to launch left-most shortcut, WinKey + 2 for the next one, and so on). But in the RC, there are a few new twists. For example, if the application in question is already open, it will pop-up in an Aero Peek view when you tap the appropriate key combination. And if that application has multiple open windows (or, in the case of IE 8, multiple open tabs), you can continually tap that keyboard combination to toggle through all those open windows, like a mini Windows Flip operation.
The taskbar in the Windows 7 RC also picks up another Quick Launch behavior: Open With. In Windows Vista and XP, you could drag a document to a Quick Launch shortcut to open it with that application. But this doesn't work in Windows 7 because this drag operation is reserved for pinning items to the button's Jump List. Starting with the RC, however, you can hold down the SHIFT key while dragging and dropping a file to cause it to be opened by the application represented by the taskbar button to which you drag.
Speaking of Jump Lists, there have been a few changes with this feature as well.
If you opened the Control Panel in the Windows 7 Beta, it sported a Spartan Jump List, but the version in the RC is populated with useful options.
The Control Panel now sports a full-featured Jump List.
The IE 8 Jump List now sports InPrivate and New Tab options in the Tasks section.
In Windows 7, IE 8 includes a customized Jump List with IE-specific tasks.