I ran into this problem when we upgraded from Exchange 2003 directly to 2010 several years ago. I’d bet it’s a general problem for Exchange 2007, 2010, and 2013 as well. Some users OWA experience will be set to the Light or Basic mode instead of the better “Premium” mode, which is the more familiar view that looks like Outlook. Other users and new users would default to the normal “premium” mode
I don’t know exactly why some users will get defaulted to this but I do know how to fix it.
How to Fix OWA Defaulting to Basic or “Light” Mode
Step 1 – Click on Options in the Upper Right hand corner.
Step 2 – Find “Accessibility”. This setting might be under “general” or any number of other places depending on your version. In 2010 it’s just under the left hand column.
Step 3 – Uncheck the “Use the blind and low vision experience” box.
Step 4 – Be sure to click the save button.
Step 5 – Sign out, close your browser then sign back into the OWA and it should load into “Premium” mode.