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">
<title><![CDATA[Comments]]></title>
<query>
<sql><![CDATA[select comments from ALL_TAB_COMMENTS where owner = :OBJECT_OWNER and table_name = :OBJECT_NAME]]>
</sql>
</query>
</item>
</items>