If you want to export the list of known issues to Excel, you can only export the first 100. This is a setting done by MS. You can change this on your TFS server. Go to "<Disk>:\Program Files\Microsoft Visual Studio 2008 Team System Web Access\Web" and open here the web.config file. Look for the setting "<maxWorkitemsInReportList count="100"/>". here you cange the value 100 to another value.
But one remark. If you set this number too high, you will encounter some performance issues.