1) EL (Expression Language) support, both for JSP 1.2 and 2.0 (selectable)
2) Compiled servlet preview. There is currently a preview of the expected HTML output, but a preview of the compliled servlet java code would be a great debugging tool.
3) Support for jsp:include and <%@ include %> in the preview.
4) Support for java refs, auto-complete, etc inside JSTL tags. (associated with EL #1)