It seems like you are hitting IE's stylesheet limitation, so some CSS rules are not honored by IE: http://blogs.telerik.com/aspnet-ajax/posts/10-05-03/internet-explorer-css-limits.aspx
Since the issue manifests after adding more controls to the page it is likely that the page was already rather complex and had a lot of stylesheets loaded on it. Adding a few more simply pushed it over the IE edge. This is probably what happens with the certain grid settings - adding a pager effectively adds controls to the page - RadComboBox, so it needs stylesheets. So does filtering - adds a ContextMenu.
Thus, the only possible workaround that I know of, is placing a RadStyleSheetManager on the page so that it can combine requests and effectively reduce the number of stylesheets loaded on the page, as explained in the blog post above.
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