There are lots of comments you can see by googling for IE7 differences.
The general consensus is to check special hacks and codes used to alter the behavior of IE6. They may still work OK or they may not!
I haven't got IE7 yet but I have removed a max/min page width hack as I can't be sure if IE7 will show it correctly (IE7 is still being amended with new versions).
Another that may need checking is a common file used for drop down menus using csshover.htc file.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.