Here is a great post from Maxime on the behavior differences between IE6 and IE 7/8 when browsing or interacting with SharePoint 2010.  The great thing about this post is that it has a visual comparison of several use cases.

http://blogs.msdn.com/maximeb/archive/2010/04/01/what-sharepoint-2010-looks-like-with-internet-explorer-6.aspx