My First Impressions:
I have just installed the latest TechNet version of Windows 8.1 (ISO) (choosing the "upgrade" option) over my existing Windows 8 Pro 64-bit installation, using my existing Windows 8 product key. (After doing a full original system backup to an external hard drive). The upgrade proceeded well, but a number of my installed programs either had to be re-installed, or repaired due to the Windows 8.1 Program Compatibility Assistant reporting "Re Install Required".
Repair Install Windows 8
NOTE: The above link was NOT inserted by myself, but by the Forum Administrator "Brink". Note that I performed an UPGRADE over my existing Windows 8.0 setup, and retained ALL my settings, programs/apps and data. I booted from a Windows 8.1 ISO DVD and inserted my existing Windows 8.0 Product Key when asked to "enter product key" early in the process. I then selected "keep all your personal settings, files, apps and data" and then sat back and waited for the process to complete. It took about 25 minutes, including 4 re-boots. (I purposefully avoided using the term "repair install" because I was upgrading from Win 8.0 to Win 8.1, not "repairing" Win 8.0 and wished to avoid any confusion in this regard!!) Nevertheless, many thanks to Brink for the above link, as it provides very useful screen shots of the process(es).
The following cases are listed for your info:
a. VMWare Workstation version 9.0.2: "Compatibility Issue". I "ran the program anyway" and found I was unable to connect to the internet. I carried out a VMWare Installation "repair" and now the program works fine, but still starts "via" the Windows compatibility issue error screen (one is, however, offered the opportunity to "run the program any way" and "do not show this again".
b. Classic Shell V3.9.5. Beta: I had to uninstall and re-install before it would work. All fine now. Note: The Classic Shell Icon has NOT replaced the Windows Start Icon at the lower right corner of my desktop screen, but clicking on that icon does open Classic Shell (In Classic Shell Setup, you can, however replace the start icon with the Classic Shell start icon if you wish). The Classic Explorer toolbar does NOT work with Internet Explorer 11. (You will see it is disabled under "tools - manage add-ons" in IE11. Nevertheless, you can enable it by un-clicking "Enhanced Protection Mode under Advanced options in Internet Explorer 11 Tools).
c. Norton Tool Bar and Norton Identity Safe (2014 edition, by Symantec): Incompatible with IE11. I presume Symantec will sort this out soon after the public release of Windows 8.1 (It can, however, be enabled by disabling "Enhanced Protection Mode" under "Advanced Options" in Internet Explorer 11 "Tools").
d. Garmin Express (to manage my Garmin GPS): It had to be re-installed, and now works just fine.
e. The first time I attempted to use Microsoft Project 2013, I was surprised to see "setting up Microsoft Project" appear on the screen. A few seconds later, however, the program opened and worked just fine. The same happened when I first clicked to run Microsoft Outlook 2013 and also Microsoft Visio 2013: "Setting up Microsoft Office" appeared on the screen, and a few seconds later Outlook opened with no changes to the original setup.
f. For those who like the old Windows Gadgets: The latest version (V8.0) of the Windows 8 Gadget Pack DOES work with Windows 8.1 Available here: 8GadgetPack - Gadgets for Windows 8
g. My printer driver re-installed itself the first time I turned on the printer.
h. My home network group had to be re-activated the first time I tried to access it and the other PCs on the network all required to download a new printer driver to access the shared printer connected to my PC.
In all other respects, my Desktop and Start Screens (including all my Windows Apps) still look the same (except for some appearance/colour changes to App icons) as before the upgrade. Windows Explorer does, however list directories and Libraries slightly differently, but not in any way that causes confusion (Woody's Windows Secrets article on the "Final Version of Windows 8.1" explains the change to Libraries in Windows 8.1.
My original boot-up option to boot to the Local Desktop and not to sign in to Windows Sky Drive/Windows Live was not altered by the upgrade; nor were my original chosen document directories.
Note that the ISO is a whopping 3.8GB in size! One assumes that the upgrade that will be available via Windows Update (free of charge) to current users of Widows 8 will be a smaller selection of only the files needed to update a particular user's OS...otherwise some may find the update exceeds their Internet provider's monthly data cap!