A recent issue was discovered with Office Web Apps and Windows 8.1.  Users on 8.1 were unable to upload PowerPoint presentations.  It would stick on Loading for ages and then eventually fail with an error.  Microsoft finally addressed this on 12th November 2013 with an update to Office Web Apps.
I can now confirm that this has officially been fixed by Microsoft.  I’m running Windows 8.1 on my laptop with Internet Explorer 11.  My Lync lab is Lync 2013 on Server 2012.  My Office Web Apps is Server 2012.  There was a patch released 12th November for Office Web Apps that definitely fixes it.
Download the patch and save it.  Then follow this procedure for applying updates to Office Web Apps server http://technet.microsoft.com/en-us/library/jj966220.aspx.
In my case I have a single server so it is as follows:
  1. Open PowerShell as administrator and run get-OfficeWebAppsFarm and make a note of the parameters -InternalUrl, -ExternalUrl and -CertificateName.
  2. Now run Remove-OfficeWebAppsMachine
  3. Install the update
  4. Restart when prompted
  5. log on after the restart
  6. Run PowerShell as administrator and add your Office Web Apps Farm again using the parameters you copied from step 1
  7. eg;- New-OfficeWebAppsFarm -InternalUrl “https://internalFQDN/” -ExternalUrl “https://externalFQDN.domain.com/” -CertificateName “OfficeWebAppsCertificate” -EditingEnabled
  8. You will see “Setting EditingEnabled to TRUE. You should only do this if users of this Office Web Apps Server have licenses that permit editing using Office Web Apps.  Continue with this operation?”
    1. [Y] Yes  [N] No  [S] Suspend  [?] Help (default is “Y”): Y
  9. Test using your internal URL eg; https://internalFQDN/hosting/discovery and make sure you get no errors in the output.
  10. Log out of Lync and then back in.  Meet Now and upload a PowerPoint presentation.  It should load.  I also copied the meeting entry info and pasted into Chrome with ?SL=1 at the end and entered as a guest to see if I could see the presentation.  And I could.
I think it is actually a bit quicker to upload and display the presentation than it was before, but that could just be wishful thinking.

Hope this helps.