Initially my external DELL monitor was not detected and I had to continue to work on 1024x768 resolution on my laptop. Surprisingly, this got fixed on it own after i restarted the system (I did boot to Windows 7 in between using my dual boot option). After that intermediate reboot to Windows 7, I can now connect my external DELL monitor and work with 1280x1024 resolution (which I could set via Setting -> Change PC Settings -> Display). Another aspect that got fixed on reboot is performance. As I logged into the Windows 8.1 the first time, it definitely appeared to be slow in responding and things like the charms bar etc. took a little longer to load. From start screen as I typed a word to search for it, the search charm took a while to load. After the restart I also saw a new background image for lock screen.
Similar to Phone 8, the tiles on Start screen now support multiple sizes, 4 to be precise. These are Large, Wide, Medium and Small. In the figure below (StartScreen.png), you can see that the Weather app is using the large size, the Desktop app is using Wide size, Internet Explorer is Medium and others like Mail, Calendar, Music, etc. are using Small Size.
Depending on the tile size, the information displayed on it, varies. However unlike Phone 8, the tiles aren't resized just by clicking and holding them. Here, the option to resize comes up from the application bar. I wonder why the experience is't the same as on Phone 8, where you see a resize arrow right next to the tile on holding the tile for a bit longer.
Tiles can be grouped in some category. I haven't done it yet and hence you can see the "Name group" option being displayed above the tiles.
On the start screen if you move our mouse, you see a down arrow towards bottom left and a dash icon on bottom right.
The arrow is used to go to screen showing all the installed applications on the machine
and the dash is for semantic zoom.
In the screen where all the applications are displayed (see above), notice that the Administrative Tools applications are also displayed. This can be enabled via Settings -> Tiles -> Show Administrative Tools option.
I had not noticed earlier but Settings shows a slightly different list depending on where it is invoked from. If from Start screen, it shows Personalize, Tiles and Help options, while if invoked from Desktop it shows Control Panel, Personalization, PC Info and Help.
The much discussed and missed Start button is back.
It looks different (see above image) and surprisingly on clicking it, you navigate back to the Start Screen, which was earlier possible by hitting the Win Key anyway. You can right click on it to see some more options (see image below) but like Windows 7 it doesn't shows the list of all applications installed. There may be more to it, which probably isn't completely implemented by now in these preview bits.
The option of going to desktop on login is also provided, but again surprisingly it isn't found in Settings -> PC Info, but rather in Taskbar properties.
The Settings -> Power options on laptop only show Shutdown and Restart as the two possible options. There is no Sleep option on laptop, though it is seen on a desktop machine. Logging off is a bit tricky. When in desktop mode, hit ALT+F4. The Shutdown Windows dialog comes up (see image below) and one of the options here is Sign out. This is the only way to logoff that I could find. Switch user is also an option in this dialog, though to switch user, you can also do something like Win Key + L to lock the screen, and then select the user for login.
The locked screen shows the time and date and also the network and battery status by default. Now you can add 7 more applications to the lock screen (via Settings -> PC Info -> PC & Devices -> Lock Screen), show their status on the lock screen.
Fast Boot and fast shutdown time is something that I had seen with Windows 8 as well and it is retained with Windows 8.1. Using Windows 7 in between clearly shows the enormous difference Microsoft has been able to achieve for this. At times I have seen my Windows 7 laptop taking upwards of 1 min to shutdown and this is more prominent when you keep using the option of sleep for many days without shutting down in between.
This is not an exhaustive list of new features in Windows 8.1, but only those that I tried in my initial run with it. If you know more, do share.