I use a user defined extension to show table comments. It works in older versions of SQL Developer, but the tab does not appear in 4.2.
<items> <item type="editor" node="TableNode" vertical="true">
<sql><![CDATA[select comments from ALL_TAB_COMMENTS where owner = :OBJECT_OWNER and table_name = :OBJECT_NAME]]>
That was a known issue, pretty sure it was fixed for 4.2 final, but I'll double check with your example.
FWIW, we do show table comments, they're on the Details page, scroll down.