This article explains the difference between OWA Light Mode and Premium Mode and why some users may only see the Light Mode client, even though they haven't selected it at logon.
Note: If you are seeing OWA light mode after applying Windows Updates, which includes Internet Explorer 11, this is a known bug. The fix is to have your network administrator install Update Rollup 3 for Exchange Server 2010 Service Pack 3 or Cumulative Update 3 for Exchange Server 2013. Have your network administrator read KB2866064 or send them this article.
Exchange 2007 Outlook Web Access and Exchange 2010 Outlook Web App offer two different modes for viewing OWA - Premium Mode, with all the bells and whistles that Internet Explorer can muster, and Light Mode, which provides fewer features and is sometimes faster. You would usually use the Light client if you are on a slow connection or using a computer with unusually strict browser security settings.
If you are using a browser other than Internet Explorer 6 or later for OWA 2007, you can only use the Light client. OWA 2010 supports the full Outlook Web App experience (aka Premium Mode) on Internet Explorer 7 and some other browsers on Windows, Mac, and Linux computers. To check out all the supported browsers and operating systems for OWA 2010, click here.
Here's a comparison between the Outlook Web Access 2007 Light and Premium clients:
And here's a comparison between the Outlook Web App 2010 Light and Premium clients:
Normally, users will default to use the Premium Mode client if they are using IE6 or better for OWA 2007 or IE7 or better for OWA 2010. However, you may hear complaints from some users that they always get the Light Mode client, regardless of whether they selected to use it or not when they logged in. This happens if the user selected to use "the blind and low vision experience" when logging into OWA for the first time.
To disable this mode and allow IE to use the Premium Mode, have the user login to OWA and open Options in the upper right corner. Then select Accessibility and clear the checkbox for Use the blind and low vision experience, as shown below.
Now have the user sign out of OWA and sign back in. They should be using OWA Premium Mode, provided they are using a supported browser.