I go all in.  Using a separate HDD, I am running Windows 8.1 Preview native as my primary OS.

Pros:

• Ability to boot directly to desktop
• Desktop image is shared between desktop mode and Start Screen
• More options with Power User menu
• Start button
• Better, more integrated search to include Internet results
• Ability to have tile icons open desktop IE
• Generally feels faster
• Better "Modern UI" Control Panel
• Integrated SkyDrive
• More locations in Explorer Navigation pane
• Better implementation of built-in Modern applications
• Shutdown items on Power User menu

Cons:

• Power User menu now shows PowerShell instead of Command Prompt
• PowerShell still much slower to respond to commands
• All command prompt commands don't work in PowerShell shell
• Still no start menu!
• IE11 is broke on some sites (why?)
• Still no Aero or Aero Flip options for primary desktop users
• Carry over, horrible flat color scheme making it difficult to find overlapping windows
• Must enter a key to install--no 30-day grace period
• No option to create local account when Internet is detected during setup
• Renaming "Computer" to "This PC"  Who's idea was this?
• Hiding libraries by default (are they on the way out?  Why?)

Power User menu now shows PowerShell instead of Command Prompt
This can be changed to CMD  Don't care.

Still no start menu!
Ah, yes there is; the same one there has always been.  Except, now in 8.1, one can select to use the Applications screen instead, of the Start menu screen.  The 'old' start menu was retired w/ Windows 7.  Personally, I hardly ever used it in Windows 7.  I don't use either screen in Windows 8 or 8.1.  Ergo, having the start menu changed from 7 to 8 doesn't matter to me, @ all.  Of course you NEVER used the Windows 7 start menu.  To admit doing so would undermine your justification of the new Start Screen.  I don't care what matters to you; it matters to me!  The one thing that--as of now--will be a must purchase is the wonderful $5.00 Start8 from StarDock.

IE11 is broke on some sites (why?)
Remember, 8.1 is a BETA.  It's not necessarily that IE11 is "broken" but, a compatibility issue between IE11 and some sites.  Likely this will not be the case once 8.1 hits RTM.  Of course it is BETA.  Still, it is built on the foundation of the previous version.  If something worked in IE9/10, it should work in IE11 now.  Of course most of the issues will be fixed in upcoming updates and in the RTM.  Still, pointing out the shortcomings is just as important now--if not more so.

Still no Aero or Aero Flip options for primary desktop users
Aeroglass has, also, been retired.... after the 1st or 2nd Windows 8 beta.  I still would like to see it come back.

Must enter a key to install--no 30-day grace period
This is not a 'Free Trial' prior to deciding to buy a product or not.  This is coming to Windows 8 owners as a Windows Update later this year.  This is just an opportunity to have a look @ what's coming before it is pushed out in its finished state.  The 30-day grace period--that is in Windows 7--allows an experienced and cautious admin to install and test an application for compatibility prior to activating.  Guess what, this is a good practice so that a key (non KMS) is not wasted.  It also allows an end user time to see if they really want to install this OS.  This issue does not only apply to Windows 8.1, but to Windows 8.  A person should at least be able to do a full install, with drivers and applications prior to activation.  Heck, I can live with a 3 to 10 day grace period.  Anything is better than forcing entering a key--and activating--prior to the install being complete.

Renaming "Computer" to "This PC"  Who's idea was this?
Probably the Windows 8.1 Build Team.  No biggie, really.  Suppose it's more specific and no chance of confusion w/ other computers, such as when on a network, workgroup or Homegroup.  Whatever.

Hiding libraries by default (are they on the way out?  Why?)
Sorry, I cannot address this one as I do have Libraries hidden by default or otherwise.  I have only found the same ole visible Libraries as in Windows 8.  Of course.  Like with the Start Menu, you never used the libraries feature of Windows 7.  I don't expect otherwise.  Libraries is an outstanding feature to have all like files in a single place (don't say SkyDrive--I guess that would be your response to that) that may reside in different locations.  This feature would be lost on those who don't use the libraries, of course.

I trust you backed up your Windows 8 so you can get it back as your primary OS when the 8.1 beta is expired.  Hahaha.  You are funny.  No, I just go willy-nilly.  I don't think anywhere in any of posts I have communicated that I do such rash actions.  Yes, I run Windows 8.1 native to bare metal--on a new hard drive (the "old" Windows 8 HDD is stored).  Data is backed up on a routine basis to a dedicated, external, backup HDD.  When RTM 8.1 releases, I will--again--install clean to yet another drive--probably my really old Windows 7 drive.  If not that one, it will be a new 2.5" drive.  Chances, I don't take them unnecessarily.

I assume you have, also, backup your data so that you can revert when necessary.

Can you do me a favor?  Send me the MSFT contact information you have so that I can also be an evangelical poster.

These are my thoughts and issues.   I have been using Windows 8 full time since the Developer Preview.  Regardless, there are a lot of things very broke.  You constantly defend the Start Screen.  Interesting as I watch person after person after person (most of them experienced IT personnel) struggle with the Start Screen.  You know what typically comes after about three or four hours?  "[*]uck this; I'm reimaging back to 7."