I would like to know the known issues in IE7 for sharepoint 2010. ? can anyone help me?
There are no documented limitations per say that I know of. See the Plan browser compatibility on technet http://technet.microsoft.com/en-us/library/cc263526.aspx. Another article I have come across in the past is http://www.milesconsultingcorp.com/SharePoint-2010-Browser-Compatibility.aspx , this one is not updated as frequently as technet I am assuming.
Where issues may come into play has to do with if you are modifying the CSS or master pages of the sites. When working with branding I constantly have to check between the different versions of IE and IE7 can behave completley different then IE8.